Although certainly not my cup of tea, sports sims are quite popular. I tend to play them not, but there was one game I was hooked at: Pennant Race 2. So my vote goes to:
Sport sims might be popular, the MSX kind of lacks really proper sport sims. The Hyper Sports/Olympics games are fun, and there are plenty nice baseball games around, but a good soccer game should be doable, shouldn't it?
A bit offtopic, but the MSX could really do with some more pinball games as well.
Meh, other than racers (which don't really count IMHO), I've never really been into sports sims very much... An unofficial port of Blades of Steel, Konami's excellent hockey sim, complete with a fighting mode would be great though. I wonder why BoS never got an MSX port. It got ported to the NES, C64, PC, Amiga and Gameboy. Maybe Hockey just isn't popular enough in Japan for an MSX release to have been worthwhile...
Best? I don't know, but I very much enjoyed playing those MSX1 joystickshaking games against friends, eg. Konami's Hyper Olympic 1,2, and Activision's Decathlon.
A bit offtopic, but the MSX could really do with some more pinball games as well. I've been saying that ever since I played Silverball, Epic Pinball and the like back in the last century.