1. CTF - I find this to be a mix of Match, Race, and Coop in the sense that ringslinger fights are more than common, you have to race back to the enemy flag when you lose it from the other team, and it requires organized team work. The CTF stages, like everything else, is nonlinear, so there's plenty of tricks you can pull in any position on your team. People choosing sides aren't a real problem, as I'm normally the one to help even out the difficulty between sides. When it's even, there's always plenty of ways to be active, be it sniping intruders, organizing strategies, flying your team's flag carrier to base, or being knee-deep in large ring fights. Either way, it never gets old.
2. Match - Match is match. Shoot others for points and avoid getting hit. There's plenty of action in this, just not as much as there can be in CTF. Only problem I've ever had is when people insist on having ringslinger 1, making it easy for me, and everyone else whining that I hog the monitor. I tell them to put it to 30, but they don't listen. =P
3. Race - It's okay, but it depends. In almost all of the circuits, it's a simple thokfest, so unless ringslinger's on, the activity's pretty dry. If it's a race through Mystic Realm, though, I'm totally for it, as you don't have to finish it just one way. You can fly through the open sky with Tails, dash through one of the multiple paths with Sonic, or do a mix of the two with Knuckles. It's also more interesting when it isn't Time-Only, as you also have to hunt for ring boxes and robots as well if you want to have a chance at winning.
4. Coop - Though I commonly prefer to neglect this one, it's really up to par with Race. I would agree with the many people in this thread that say you shouldn't just treat it like Race, though I've often done so before. Coop means cooperation, and even if that is easy for some people, it's great practice for the newbies, and if you add a real twist to it (for instance, I often load aggressiverobots1-4.soc along with a level pack whenever I host coop), then you'll really have something going.
5. SP - Eh... technically, it isn't bad, but I sort of prefer hosting private, unjoinable netgames when I play something alone, as it adds more freedom to the game. (Skipping to Arena Zone in Mystic Realm, loading Aggressive Robots soc, turning ringslinger on... yup, fun times when faced with boredom.) When I want to save my game, though, I'm more than willing to just go with Single Player at that time.
6. Tag - Um... yeah. The more people there are, the easier it is to not be tagged. That being said, tag is boring. >_>