That's a bit like asking, "what are the best spices to make food taste good?"
There are so many choices. For small tanks, I like clover fern (Marsilea crenate) for a "carpet" plant, Salvinia natans as a floater, and various cryptocorynes. C. wendtii looks especially good in a small tank. If you like grassy plants, try some dwarf sag. Java fern is nice for giving hardscape a more natural look (I superglue it to wood and rocks), but it can get a little big for a 3 gallon. Mosses can do the same thing: Java moss is easy to grow and looks OK, but flame and Christmas moss look nicer.
But it's true that a 3 gallon is really too small for anything except inverts. Invert tanks can be very cool, though. I personally want to do a planted 3 gallon with red cherry shrimp, copepods, daphnia, seed shrimp, scuds, and whatever other micro critters I can get going in there.