Which one of these systems is the best for homebrew game development?

I have hacked games for most of those (not done as much PS2 as others).

For the purposes of development I am not sure where you got your information, or at least it is potentially very misleading.

Ease is generally governed by what you have to program it in and how meaningful those options are (some things have proof of concept or limited options).
Anything that is older than the GBA for handhelds or older or Xbox for consoles will generally see you want to play mostly with assembly language, as opposed to C and maybe C++ for some of the newer stuff
A few have interpreted languages as well (usually Lua, though python occasionally gets a look in. https://gbatemp.net/threads/attempt...amming-languages-available-for-the-ds.357792/ if you are bored there).
https://devkitpro.org/ covers several systems and mostly does nice C and assembly with libraries and example code.
For the GBA then http://www.coranac.com/tonc/text/asm.htm might be worth a read.
PSP will have a bunch of stuff too.
PS1 and PS2... better than the 8 and 16 bit era stuff but nice refined homebrew dev kits (as opposed to old compilers and leaked SDKs) are going to be a bit thinner on the ground.
As far as assembly goes I actually like newer stuff. Sure 6502 (NES. http://www.obelisk.me.uk/6502/instructions.html ) and Z80 (many things but at least the sound aspect of the megadrive, main thing on master system and sort of main thing on the GB/GBC) have far fewer instructions than newer stuff like the ARM in the DS and 3DS (never mind modern PC) but it is also more limited. On top of that older system tends to have more hardware limitations -- sprites might have to be fixed sizes, colour palettes are more limited, memory is more limited, storage is more limited, resolution might be more limited and not be "whatever your imagination wants" like later stuff where it was almost arbitrary... (can I hit the limit for GBA memory, sure http://pineight.com/gba/managing-sprite-vram.txt but for hard practical purposes you can fill the screen and have it all jiggle quite happily). That said others do prefer the lower instruction count stuff over more modern (for reference I learned assembly on the PC and ARM based things so had complexity there from the start).

We can discuss the relative difficulties of assembly or hardware setups more for those if you want but I will skip that for now.

From those if we are taking out anything you have to develop in assembly for, and stand a chance of finding active communities of a sort to ask questions of
DS: old but has one of the best homebrew scenes going with lots of people that know it.
3DS: Android and IOS rose up and more or less killed the DS and PSP scenes as far as them being the main active places where people doing homebrew would do their bit (for the time they were best in class handheld devices) but never the less it is like a faster DS for many purposes with a nicer 3d setup. Has a fair few people that understand it and might be able to answer questions.
PSP: Considerably more powerful than the DS (see the emulators it handles and relative quality differences even in spite of the PSP scene largely not getting down and dirty with the hardware like the DS peeps) and has a few more options as far as libraries (its SDL library being among the more notable).
Wii: Not so bad when all is said and done. The powerpc and some interesting hardware choices, as well as relatively weak hardware, can make things more fun than doing something for a contemporary PC or something like the xbox 360 or PS3 but you can make an absolute monster of a 2d game quite happily on this and its 3d is new enough that is not weird and esoteric like the PS1 and DS.

Only other thing that really ranks up here for active homebrew scene would be the original xbox, which is basically a pentium 3 PC and has the perks of being that (though most will probably still opt for the leaked SDKs rather than the homebrew ones). Sure the gamecube, dreamcast and various other things have nice examples of homebrew but in terms of raw development at the time and since... no. Not sure why the PS3 and xbox 360 did not take off to the same extent (PS3 hacks came later and homebrew 360 was a bit tedious) as they would have had some lovely stuff if it was embraced.

 

If you have a rgh xboxx 360, you could look into rgloader