I'm going to go with the soldier. As a large back and slowest moving target he is a prime target for spies, the helmet and slowness makes sniping him easier than many other classes. When I snipe or spy I tend to kill Soldiers the most. To take down a well placed (or even moderately placed) sentry they tend to need medic support. Also, if the sentry is being tanked they need a crit rocket to come along. While it does turn up eventually the slow rate of fire means that another defensive class like a Demoman or defensive Pyro will eventually take you out. The soldier is really the jack of all trades in terms of combat and is unable to dominate any particular class. This means that despite his combat weaknesses he doesn't necessarily out perform any other class. His health bonus does help in combat but his slow movement and larger frame mean that more shots are going to hit home and that the Soldier can't easily dodge as many attacks as other classes (except the heavy).
With a medic they are a force to be reckoned with, the destructive power of a Demoman and the stamina of a heavy. With a good medic they are very difficult to kill, push through the enemy line and can cause a lot of building destruction, especially with an ubercharge. However he has weaknesses against infantry even with a medic. His missiles are easy to dodge and his slowness makes it easy to get around him or destroy the medic and even if he kills you he should be killed by someone else straight after. Even with a medic the weakness against spies and snipers is still there, if not more so because a duo is often targeted more than a single person.
The missile launcher is alright if you know what to do with it but slower and cumbersome to newcomers. However it fails in one on one combat, usually because of it's slow rate of fire and the enemy ability to dodge it. If you miss once than you're open for a short time, and it's easy to miss. In open spaces it is far to easy to dodge the missile and eventually bring the soldier down. I would expect for the Shotgun to be used more often with the Soldiers but for some reason it rarely is. That thing can kill a few classes in a couple of shots, it can down anything with 125 health in at least three shots, if not less, and has a higher rate of fire than the missile and cannot be dodged. I see plenty of soldiers using their missiles against a moving target, missing every time, when they could be hitting with the shotgun. The missile seems to be good for taking enemies from the flank when they don't see it coming, but a good player wont leave themselves open like that very often.
The soldier has some good aspects about him but these seem to rely on the planets being aligned and don't outweigh the bad aspects. In the right hands a soldier can destroy everything, I've seen it happens, but it's easier for other classes to fall into the right hands than the Soldier.
I play on the Xbox 360 version so possible updates might have changed this.
Huh, I think I pay too much attention to things some of the time.
