We never quite saw the ultimate one, and design decisions vary as well.
For the sake of those joining us then you can't play GBA code from a DS slot (it is too slow). There is limited proof of concept emulation in DS mode but you are mostly then limited to having a GBA flash cart (see also GBA expansion packs like the 3 in 1), DSi mode has some better options or a DS flash cart featuring its own fancy CPU (which mostly means the DSTwo as the others with CPUs have less developed things here). No emulator beats hardware, and naturally the failings will come in some of the more noted games in the GBA library, but emulation on the DSTwo (and to a lesser extent iplayer and ismm) will still allow you to say you did for the vast majority of the library, and have some nice perks like turbo things and button remapping without modifying the console or hacking the ROMs on an individual basis. The DS and DS lite featuring a GBA slot also allow you GBA flash carts, for which there are now some very nice ones and frankly anything that was not a supercard will probably allow you to play everything you want if it fits on the thing (there are some really old or esoteric things that are smaller than the largest game ROMs).
For a good candidate then most would probably go for the Supercard DSTWO. It played all the ROMs, its hardware quality was as good as anything else/more than acceptable, it stuck it out to the end of the DSi/3ds updates that cared about flash carts, it arguably had the best ROM altering features (its savestates were not necessarily orders of magnitude above others, its guide function was lacking compared to some but ultimately still enough, cheats probably pale against AKAIO but still cheats and also the ability to search for them even if that was a party trick rather than something someone would seriously use over the other methods), DLDI was DLDI but all the enhanced homebrew* (including video and way more than passable GBA emulation in DS mode with all sorts of perks you get from emulation) is both there and very much of note. Its main issues were the expense (I take it this hypothetical is money no object) and power drain, as well as becoming rare as rocking horse shit towards the end there (and it continues to be similarly rare to this day). While it was going it was one of the best things at having new games work before an update came out to fix the anti piracy but that is obviously years in the past now.
The DSone/DSOnei was its predecessor and is largely the same story but without enhanced homebrew, cheat search, GBA emulation and video options but also a sensible power consumption. We have seen a few clones appear of late but build quality/testing done is quite variable
https://gbatemp.net/threads/dsone-clones-appear-to-be-available-for-purchase.586226/
*enhanced homebrew was technically seen in several other places but nothing was really done there other than the iSMART MM (a sanctioned flash cart seller's version of Supercard's own iplayer with an older DSTwo loader ported to it but agreement with supercard meant no real homebrew got made for it) having a version of Dingux (Linux based emulation setup, think retroarch before that was a thing and actually useful) on it. Very few will have used dingux in anger but it is technically just about able to run PS1 games at low framerates on a DS and some of its 16 bit efforts were in some ways better than some other things.
AKAIO, the sort of maybe open source answer (AKRPG was open source, closed at one point and then became AKAIO for the AK2i series and wood for what wood works for) then forms the main counterpoint from those in the know.
The average flash cart team cared about having ROMs work, and often borrowed from these guys who often made the fixes and sorted things out. Their cheat options were some of the most stable if you were the kind of guy to ride it hard rather than just needing "write this area until the end of time as I want infinite money/ammo/health/...".
The main problem is most of the extra fancy features seen in other flash carts were lacking here, and AK2i build quality was incredibly variable -- some will still be running in 50 years, others did not make it out of the box. If you care it also has some of the nicer support for the 3 in 1 (popular line of GBA flash cart/GBA expansion packs) seen in firmware anywhere (indeed I might even place it ahead of EZFlash's own mainline options) but there is plenty of even better homebrew if you want to go that way.
This also informs most of what goes for the R4s that are wood compatible. Assuming you dodge ones set to time out after certain dates (see timebomb) then most of those will lack nice savestates, in game text guides and such.
Team Cyclops then with the cycloDS.
I don't know if this made it to the end, however it is the only thing with DSi functionality (don't think even the supercard 4 in 1/DSTwo+ did much here beyond boot in DS mode and 3ds mode) which means I think you can play DSi games and DSi enhanced games (mostly a few things got a lame camera option, WPA wifi security and that is it really) as well as the actually increasing amount of DSi homebrew. The DSi was generally a failure though so nobody cared about missing extras from it really, and homebrew never took off in a bit way. Today most will probably be using the software methods.
Oddball choice. I will stand up for the EZ5i here from EZFlash. While it was current it formed a real counterpoint to anything else (good/fast updates, nice features, build quality was there). It was one of the casualties of the big flash cart killer updates for the DSi/3ds. This was also before the DS stopped seeing game releases finish (pokemon conquest was a few days later but you could find a patch, and pokemon black and white 2 coming after this. It is also not a hard cutoff -- plenty of less notable but still good stuff from after it will still work on stock) which means in a technical sense the compatibility is not as high if you look at everything in the library, look at what came before that point and it was up there. Not sure what goes today with the independent libraries of AP fixes but probably could get it going on.
Its features were up there with the DSTwo aside from the enhanced homebrew and cheat search. Its in game guide function was better by dint of allowing for a tiny bit of markup, its cheat engine apparently could be ridden into the ground more easily than some others but was still fine for 99.999% of uses. It had some of the nicer anti anti pirate** features as well -- below 8000 and some of the better stealth options were seen here, and meant it could do as well as some during the pre patch days. Its DLDI read/write speeds also got to be pretty impressive as it onboarded some things (technically making it enhanced homebrew but... yeah).
To that end if you only care about running it in a DS/DS lite or suitably modified 3ds/R4, and the handful of games released after the flash cart killer update you either don't care about or can use the external libraries to get going on then you can have a really good time here with this one. I have wondered if they would come back with an update like they did for the EZ4 to finish out the library but so far no, and have not open sourced it either like their other devices have since been.
Everything else is mostly meh, similar story with the flash cart killer update or made work by virtue of external things. This would include the M3 family (which gave rise to the original R4*** and also G6 which has some perks, also screwed over a prominent homebrew developer but you can read the dramas of sakura on your own time and learn why you need a patched version of some homebrew to work on it), AKRPG maybe (it had some onboard storage things that could pose an interesting question). M3 also seeing the only real other choice for an expansion pack for GBA slot fun here.
You can dip a toe into the sea of R4 clones and things with R4 in the name as well if you like. Build quality is variable but if you dodge timebomb and get one made after the flash cart killer update then if your goal is play all DS games with cheats, soft rest and minimal hassle on DSi/3ds (in DS mode) then you can certainly get it going on, naturally I would suggest one officially blessed by Wood rather than using a hacked version thereof. Wood aside the anything with R4 in the name (give or take maybe a few clones of better flash carts mentioned elsewhere in this) is the TV dinners of the DS flash cart world -- it is not fine dining but you will not starve on them either.
One day I hope to see something like we saw with everdrive in many consoles and everdrive and ezflash on the GBA and GB/GBC but for the DS. The enhanced homebrew thing makes this tricky in some regards as there is actually some good emulation from that, and despite my dismissal above there is a handful of DSiware that might be worth looking at (granted there is softmods for that).
**relevant
***for those unaware the R4 came from the same factory as the first runs of the DS slot M3. For my money it was not the best flash cart available at that point but it was very much a drag and drop affair (every new ROM needing an update unlike say the earlier EZFlash 5 family where you had save lists that meant a savelist kernel would run ROMs from years and years later) and cheap so became the sales winner of flash cart world, and thing everybody competed with. R4 then became the, as much as I hate to use the term, normie term for flash cart much like you might hear jailbreak or data miner when they mean hack/hacker (or if we are going more old school then the stereotypical 90s mother might say playing Nintendo/Nintenda or playing Sega) and thus we have a million things using R4 in the name.
Thus ends the abridged and cynical history of the DS flash cart scene. You would be missing Golden Sun Team, the DSX and Ninja DS, as well as TTDS/DSTT/ysmenu from the more completionist version but that is for good reason. Also it would suck but if it is a locked in a room with a DS and DSTT/ysmenu affair you could play the DS library.
