I think RA2 was the most balanced of the original 4 games for multiplayer, and some of the units are just great fun. I particularly like the spy tech units. The Chrono-commando was just hilariously OP, but at least the game would be pretty much over by the time someone managed to get one
Firestorm for me is the one. It has the most interesting world and a great set of units and buildings. I didn't get to play much past Generals Zero Hour though.