I'm split in between both, while I always wish for BOTH a good single-player and multiplayer experience, I want the game to be a good experience in either one so that it's replay value is higher and it will make want to come back and replay the game again from time to time. For example, games like Halo 3 and CoD4 have a stronger multiplayer component than single-player, but the campaign is still excellent for both. However for single-player only game slike Oblivion and KOTOR II, they are such excellent games that they can stand the test of time with just a single-player component, but multiplayer is what really keeps a game fun to play for years to come.
Either way, I value both single and multi equally.