I don't know about Donkey Kong Country 2 & 3 but Super Mario Allstars + Super Mario World runs pretty well. The first Donkey Kong Country ran pretty fast (Last I checked) so you might wanna put "Wait for vblank" in the options setting.
Here are some games that I recommend that actually works on SNEmulDS v0.5a2 (Alpha 2 just so you know). And btw, all of these are the list of games on my 1GB Kingston MicroSD right now so yeah, I know they work well.
1. Mega Man X - Works pretty well as far as I know. It has some sound issues on the highway intro stage so if the sound stops, just hit debug and then back. The sound should start working again.
2. Earthbound - Play this if you haven't tried already. If you are new to the game, it might take a bit of time to get into this strange RPG, mainly because the battle system turns people off but if you manage to adapt to it's look and feel, it's one hellavuh good RPG.
3. Aero Fighters - A good air shooter and a classic too. You might need to adjust the screen and possibly layers to get the best view.
4. Super Contra III - Best in the series IMO. It's pretty challenging, especially on Hard Mode. It has minor layer issues so you'll need to adjust it (Mainly on the first stage and bosses) to view some invisble stuff like the tank and the enemy tank on the first level.
5. Super Bomberman series - Packed full of fun and explosions. Only get 1, 2 and 3 because the 4th and 5th have graphical errors that can't be solved at this time with layers and other stuff.
6. Zombies Ate My Neighbor - Really good game, and very long too. Basically, you rescue residents and stop them from being killed by zombies of course. I haven't really played that far into the game so I'm not too sure about any type of issues.
7. Breath of Fire I & II - Not too sure about BoF II since I didn't play pass the prologue (Wanted to play BoF I one first) but BoF I has a bit of layering issues. Not too much of an annoyance but anyway, very good RPGs.
8. Goof Troops - It's actually pretty fun but too short. No real issues as far as I know.
9. Rocky Rodent - Fun platformer while it lasts. Minor layering issues but works pretty well.