Sandman is not fun, as it is the best. = /
Edit: I'm just going to go through all the secondaries (all based on my experience):
Sandvich almost always better than shotty.
Sasha ♥ Heavy, Natascha is a whore.
I use fists for the kill taunt. >.>
Kritzkrieg is situational (and normally only better if you're team is steamrolling the others), and the other medic weapons are both easy upgrades (go go combat medic!)
Scouts with the sandman seem to help more overall, but they'll get less points as they'll probably die more.
Force-a-nature and bonk seem like silly toys, though FaN can help with speed capping the last point on CP maps by blasting people off it.
Backburner sacrifices too much by losing the airburst (saves engy buildings, medic buddies, and yourself, all the time.) Axetinguisher eats baby...-eaters (heavies)
Flaregun's best for arena (damn pyros), but the shotty works better in normal play to take down people that run off after being lit up. And if you're just taking pot shots from far off at people, I hope you get sniped.
