Dont bother with pipe bombs if you become great with the stickies they're all you need you get 8 of them and they seem to reload faster too
Think three steps ahead; Where is he? Where is he going? Are you close enough that he's not going to be able to change direction in time or do you need to factor that in?
If fighting them from a distance I typically just lob it at their body and blow it up soon as it hits them I can't tell you how many times someone has found out that just because they think they're at the end of the sewer tunnel in 2Fort and i'm at the sewers that they're going to reach the health in time.. thing is I can hit that health from the sewer tunnel and soon as they predictably go for it they're dead
When fighting a medic'd heavy put about four or five at his feet while leaping around like a retard I promise you he'll die before you once you get it down and the medic 9 times out of 10 will run soon as he realizes his little buddy is nothing more than little chunks
With a scout they're almost always going to try to get in your face and bat you to death just put a bunch at your feet and blow them soon as they're in range you deny them a point and force them to respawn too which is better than you just dying alone
Dont worry about a pyros air blast most of them just run around kekeing about their +50 health and nub crits on backburner

same tactic as a scout blow them and yourself straight to hell if you have to altho its less likely to be a neccessaity with a pyro because most times you can just nail them with a few stickies and finish off with a pipebomb to face
Spies are easy just bottle feed them most dont even realize the revolver is one of the best weapons in game (imo)
Treat medics along the same lines as a pyro i'd say about 8 or 9 times out of 10 they'll shoot you a few times and then instead of reloading just run in to try and bonesaw you; you can typically pipebomb them easily because they'll come at you in a straight line and only a cross eyed retard could miss that....
In my personal experience demos are the easiest because most can't aim for crap so I just typically get in their face and bottle feed them not worth wasting ammo on.
Snipers? pssh... hide and then full charge+hold reload or secondary attack and then jump out and lob it across the map into their face one is usually enough to scare them off long enough for you to get in range while they're med cabneting and then they get to come back to you in an ideal range to laugh as you pipebomb them to death because hey most snipers will *still* try to snipe you and what do they do when they do that? most stand still
Not even going to bother describing engineers because they're a null factor at best to a good demoman
Soldiers... its all going to be luck there because they crit out the ass and can just stand in your face shooting at their feet or if they're at a distance you better hope you're 30 feet from the crit or it'll instakill you

typically if i'm outdoors i'll sticky jump away and then take them out from range and height