Halo and CoD4 had both great single player and multiplayer, so I liked those. Orange Box balances by splitting the games, which could be interesting for other series to try and do. Make a SP in one universe that wouldn't be a good MP, like say Bioshock, then use the engine in a different setting for MP and maybe keep some of the same ideas (Like TF2, you could spec out different plasmids or gun upgrades...)
Crysis has a GREAT single player, the mp is tacked on "eh" to me.
MP does make the game if it that's what it's made for. CS:S, TF2, UT3, and more.