This one can really divide players, and really is driving the industry right now. But in the end, which do you like better: Single-Player gaming, or Multi-player gaming?
I have to say single player myself. Back at the turn of the millennium, people did not have a reliable internet connection. What was forced to be? Great games with a great story, characters, atmosphere, etc. The kind of games that you can enjoy almost any time, and you know that they will be good. Even today, all the big name games all started out as single-player wonders (even if they jumped the shark afterwards). Here are some examples of great single-player games that jumped over to multi-player when the internet improved: Halo, Call of Duty, Command & Conquer, Battlefield (you can argue that it was multi-player to begin with, but the bots got worse over time - just look at BF 3 compared to 1942) and many more that I have't played yet.
As I like to say, I have a life other than just video games, so the argument of "just play more and you'll get better" won't work for me, as I don't have time to get good.
And my biggest grievance with multi-player: more often than not, you will come across cheaters and grievers who make the game unplayable, no matter what the game.
Yes, multi-player can be much, much better than single player, but this is how it goes (at least for me): Multi-player has truly awesome moments every now and then. They last a short time and then are gone. In between those moment,s you have normal game-play, all the way down to the cheaters and grievers.
Single-player usually cannot compete with the pinnacles of multi-player, but it is much more consistent. I find that I prefer the great (note, not super-great) game-play of a single-player campaign to waiting for that next multi-player crowning moment. So on average, I enjoy single player much more than the internet has to offer.
And sort of tacked on: Not really a problem for me, but it sometimes is, and I know people who do suffer from it: Multi-player has to have you connected to the internet. No reliable connection? Can't play! Single player doesn't have this problem.