I would say MP. I was at one point in time only into SP. Never cared about MP. The world n I have changed since the Commadore 64 n NES days. The internet has brought people together. I don't think we should have to choose though. Game companies need to do a better job at developing a game. From sp to co-op to mp. Singleplayer needs a good but longer story. The technology exists to make games the way people want but because game companies tapped into the console users they realized that crowd will buy just about anything. It's the only reason CoD is still around. PC users voiced there opinons. While some still buy PC games like CoD, most will just cheat or pirate because of the crap console ports they have become. PC users should NEVER have to settle because a game company doesn't want to make a actual PC version. Lets not forget that if it weren't for PC, CoD wouldn't even be around nor would the billion dollar sales the game has generated each of the last few years. All I'm saying is build a game for each. Consoles have there place n will never be as good as a PC, ever. PC users need but really deserve deticated servers. It will and has always stopped/limited cheating.
Basically, make a game that has all 3 parts, sp, mp and co-op. Not many games have all that work good. BF3 has co-op but it's only 6 missions/maps. That's terrible and even after spending another $50 for a map pack that was a rip off from the beginning, no new co-op missions were added. What a waste for a game. Could have had great co-op play but got over looked for mp which while fun, isn't the best.
Single only. My partner and I never have and never will play multiplayer games. We've zero interest in playing with others (oo-er missus). For us gaming is about relaxing and getting away from people and real life! Also we don't tend to play in a style that would work with other people.
We play all different games (Crysis, The Sims 3, Settlers, etc) and never use ANY of the online features at all.
We also like to be able to use cheats as and when we feel the need.
If all games went mutiplayer only then that would be the end of gaming for us...