The Nintendo cards are the best, but later MadCatz and Nyko memory cards do well too.
I have a half dozen of the 4x MadCatz Memory Cube cards from late in the gen--they are solid plastic (not transparent or translucent) with a small cube embossed at the top near the port, the logo is painted on, and they have ridged finger grips on the sides. They came in bright colors--we have green, pink, blue, and orange. I think they had a white one as well. We have had them from 4-6 years and no problems. Target clearanced 'em out cheap a few years back, and I regret not buying a cart full of them to save for gifts.
I also have a 128 Mb Nyko Memory Card. Its Black with a red rubberized square in one corner and a white Nyko logo. Finger ridges on sides, and one corner is cut off, Wii style. Had it for 4 years, never had a problem.
If you need wireless, get a Wavebird. If you don't have the money now, pick one up from Gamestop when you can afford one--they usually have one laying around. They are very, very well made, very durable, and well engineered. Damn shame the big N did not give them rumble.
If you have a Play N' Trade around, they tend to have a wide selection of cards--they are the best place to buy Sony branded PS2 cards for a reasonable price.
Edited by donssword, Sat Mar 19, 2011 11:34 PM.