Ensign definitely, would also happily under go prosthetics.
TOS - Has to be a redshirt and then die heroically.
TNG - Standing at the back trying to figure out exactly what the hell Mission Ops is there for.
DS9 - Would want to be everywhere on the station, runabouts and Defiant
VOY - Would fancy one of the side stations on the Bridge, and of course on my last appearance be sporting lieutenant jg pips, just to stick it to Kim.
ENT - In engineering, to swoon over Trip all day.