I don't have a favorite weapon, but there are several things that culminate into a single philosophy: it should do what I think it does. As much as I love TF2, the weapons drive me nuts at times, as they did in Counterstrike. A shotgun should be useless at long range, but up close it should be an instant gut full of lead, even if you aren't spot on target. A sniper rifle should be a definite kill if a major body part is hit (bonus points if hitting an appendage is non-lethal but visceral and debilitating), but it should require patience and aim to get a good shot off (hard in games where everybody just runs around randomly anyway). An assult rifle should have enough accuracy to get a few shots in at long range (but still useless against a sniper), while being quickly fatal closer up. A fully automatic weapon should have enough fire power to kill anything within a medium range, but a stray shot shouldn't be more than a flesh wound. A machine gun at point blank had better beat a maniac with a knife.
The biggest thing that is important to me is balance and ability. Detail is always nice, including crisp, clean models, powerful sound effects, and (for futuristic/fantasy weapons) flashy visual effects. In the end, I just want to be able to kill something.
As an example of weapons that DON'T work as expected:
- The knife in almost any game is an instant kill. I'm often killed by people running directly at me with a knife while I spray them with a shotgun/assault rifle/machine gun. It makes you wonder if your gun is actually working.
- Most shotguns are far too underpowered at close range. I don't expect to pick people off from 50 paces, but when they are close enough to knife you, they had better splatter when you fire, whether you are pointing at their head or their legs.
- Sniper rifles that drift, bob, droop, and still don't kill a person in one shot. Some games will only give an instant kill for a head shot and these can be nearly impossible to get. What's the point of a sniper rifle if you can only injure somebody before they wake up and start running around like an idiot. Sure, make it hard to hit, but make it worth the shot.
- Various weakly-hitting machine guns that still kill from long distance. If somebody sprays an Uzi at me from 500m, I expect to catch a bullet now and then, but I really hope that one random shot won't hit me in the head and drop me. Perhaps in real life, but then again it doesn't take 5 shotgun shells from close range to drop a person either. Long-range spraying should only be viable if enough bullets fly through the air, not if one just happens to hit the right spot.
If all the balance is taken care of, go to town. Plain old handguns, machine guns, assault rifles, and the likes are great. Big weapons, small weapons, they all have their uses and can all be fun. I like silenced pistols, massive miniguns, rocket launchers, sniper rifles, and even an axe now and then. Futuristic or fantasy weapons can be fun too, like a flamethrower, rail gun, BFG, or just about any other particle effect maelstrom.
As long as I don't get the feeling that my weapon is what's making me lose, I'll take anything. It's just that much more fun when you have quite the variety to pick from.