Here's my loadout:
Soldier's my main class, so I can give you a pretty good idea of what rocket launchers to use when.
Direct Hit: I use the Direct Hit the most (as you can tell by the Hale's Own DH in my loadout). I find it works best when you run into a lot of one-on-one fights, like in 2fort. It's also good if you're fighting a lot of heavies...if you hit with all four rockets at close range, you can take out an overhealed heavy. Plus they're easy to hit with the DH. I almost always carry a shotgun instead of a banner - the DH does a lot of damage to you at close range, and if you miss you'll need that secondary weapon. But, it does charge the Buff Banner pretty fast, so sometimes I'll use that if I'm getting enough support from my team.
Regular rocket launcher/Original: My second favorite rocket launcher. A good all-around choice, works well with the banners or shotguns. One downside is that the rockets are slow, which can be a problem if you're going up against pyros that are good at reflects. I usually use the Buff Banner as my secondary with this rocket launcher. However, it can be hard to kill heavies, especially if they have medics. If you have cover to retreat and reload, you can kill them, but out in the open you're in trouble.
Black Box: This rocket launcher's good for when you're having trouble staying alive. So, if you don't have a lot of medics or dispensers, it can really help. However, it has the same downsides as the regular rocket launcher, and only three rockets. Although, the three rocket limit tends to teach you how to conserve your rockets. I usually use the Buff Banner or Battalion Backup with it.
Liberty Launcher: The increased rocket speed is good if you're attacking at long range a lot, or if you're going up against pyros who can reflect (although if they're really good, they can still reflect your rockets). Not a bad choice, but not one I use regularly.
Cow Mangler: Pretty much useless. Although it can't be crit boosted, it can get mini-crits from the Buff Banner, so a few times I've used it along with the Buff Banner to decent effect. The not requiring ammo part could be useful if you're on a map without a lot of ammo and you don't have dispensers. Not being able to kill sentries with it is a big downside, and the disabling effect of the alt fire is hard to use and doesn't last long.
I don't use demoman that often. I can say that a demo that knows how to use his stickies properly is very dangerous. So, practice getting the timing down on the stickies, and learn how to detonate them in air. Practice aiming with the grenade launcher too - direct hits with a 'nade do a lot of damage. Demoknights are fun, and good for one-on-one fights, but are pretty much useless for anything else. Although, their charge and damage resistance can make them good flag carriers, too.
I particularly like the Dead Cone hat and Foster's Facade, added to my inventory when I bought Plants vs Zombies and Killing Floor respectively.
Will occasionally go medic just to support a heavy. I love the Bill's hat that I have on my heavy, too. 
