All the major brands are good, I tend to avoid the fluval ones, to many compartments for my liking and you dont need the carbon pads in planted tanks so its a bit of a waste. All the eheim ones are good, as are the tetra and rena ones. Get one with two or three media baskets and fill them all with sponge.
Sam
EDIT - oh and its always better to overfilter a tank, so rather than getting one thats rated for tanks up to 20g, get the one rated for tanks up to 40 or 50g.