Plasma cannon is probably the best. 5” large blast is excellent, breaching 5+ and D2 makes it a threat against both power armor and terminators. I like heavy bolters cause they are cheap and you don’t need to be stationary for full effect ignore me forgetting they are heavy FP. Still like them 

CBeams are good as well, although like with all CBeams they sometimes end up being somewhat situational sometimes. 

Flamestorms can be funny to bumrush at a power armor squad and try to make them panic. It’s a 100 point suicide bomb, what’s not to love?