Hmm... A buttstomp is good, and does as much damage as a powered weapon, but it takes a while to use. On the other hand, it can be quite useful, as it lets you bounce off enemies and other players to reach high-up places. An uppercut is usually considered useless, but if it's used just right (like while ducking in a hole and someone's about to stomp you) it can be a nasty suprise. If a level had lots of one tile holes in the ground (castle roofs, anyone?) and maybe no-fire zones, I'd definately want to play as Jazz. A sidekick is incredibly useful, but hard to control. In single player, this is practically unstoppable. However in multiplayer, unless you're suprising your enemy or have some ice handy, you'll probably get shot while kicking. Lori's karate thing is about the same, but doesn't go as far and is a bit wider. I can think of some instances where this would be preferable... but not many. A turtle shell is also useful, but doesn't do much damage and is rare in multiplayer games. Since they can only move down and straight, I see them as an extra 100 points in single player and something to distract the enemy in multiplayer. A bird... these are alright, as they essentially give you an extra health point (four HP can be a lifesaver in CTF, as I'm pretty sure it absorbs all the damage from a powered weapon, making it five) and are alright weapons in singleplayer. In multiplayer you need a(n?) MCE orb to get them to attack players, and those orbs are even rarer than birds and tend to get "lost" during teleports. I'm not sure how a chuck bird would work in multi, but in MP a bird's best use is one or two extra HP (though don't you feel bad using the poor creature as a shield?). These seem to have been degraded to pets on hotel levels.

Overall, I'd have to say buttstomp. Though it is slow to load and sometimes easy to uppercut, it does two damage and can help you get around, not to mention it's available to all the characters (except frog and bird, of course).