This info from the TF2 Wiki seems pretty relevant to your question.

The only classes that you can't kill with a zero charged headshot with the sniper rife are Solider, Pyro, Demoman and Heavy, with each of these requiring 17%, 9%, 9%, and 50% charge for a one shot (headshot) kill.

Also worth noting that you can oneshot kill (bodyshot) a Scout, Engineer, Medic, Sniper and Spy with a 100% charge if you think it's a better bet than the head.

All of this goes out the window if a medic is charging, but the above link has info on that too.

This info from the TF2 Wiki seems pretty relevant to your question.

The only classes that you can't kill with a zero charged headshot with the sniper rife are Solider, Pyro, Demoman and Heavy, with each of these requiring 17%, 9%, 9%, and 50% charge for a one shot (headshot) kill.

Also worth noting that you can oneshot kill (bodyshot) a Scout, Engineer, Medic, Sniper and Spy with a 100% charge if you think it's a better bet than the head.

All of this goes out the window if a medic is charging, but the above link has info on that too.