I think the worst is door killed... Its possible to evade just about any of the others, heli kill, DB.. getting rammed is even easier to evade.. the firetruck may be a little annoying but its your fault if you didn't see it coming....
but... getting shot in the back by sawnoffs as you enter a door is probably the most annoying thing ever... you die even though you made it... its a cheapass shot where the other person has no way of evading it, and it takes off quite a lot of health/armor and the person usually dies after making it inside..... in fact its so lame i think it should be banned like door camping...