I use a SanDisk UltraII 4GB, which apparently is one of the fastest/most stable. There's an ExtremeIII version now, but I hear that's only in Sweden, and expensive too.
It's been a long time since the last memory card debate. I think the most preferred was Lexar PlatinumII, but I can't remember if that was for DS (MicroSD) or MS Pro Duo. I'm too lazy to check right now, but if it was MSPD, then either Sandisk UltraII or Lexar PlatinumII are the best bets. Also, don't go for the Sony MarkII, as those are found to be the most common fakes in the market, and didn't benchmark nearly as high as the others.
Then again, the difference might just be a couple seconds in loading times. Even a MicroSD->MSPD adapter works fine, even if it's somewhat slower with a much lower lifespan (the smaller it is, the quicker it'll die). But you'll be bored of your PSP/DS long before your memory card dies on you, so no need to worry there.