In the finale? Oh, I don't know. I would have suggested killing off Travis at some point in the show's run, if only to trim some of the deadwood. For the grand finale, though, I'm not quite sure: one could suggest Archer, although they did (sort of) kill off the captain in the finale for DS9, so it could seem a tad familiar in that respect.
Honestly, if the death was compelling and made sense from a narrative viewpoint (so, basically, the opposite of what happened to Trip in "TATV"), then I would be okay with them killing off anybody. For the maximum dramatic effect, it might help if it was one of the characters that actually got some decent screen-time, which is pretty much just Archer, T'Pol, and Trip. If the other characters had been developed better, then maybe Reed would be the best option (perhaps sacrificing himself in the line of duty, which would be fairly in character); Hoshi might work, if only for shock value; Phlox I think would be the oddball choice. And then there's Travis, of course, but would anyone really care?
Honestly, if the death was compelling and made sense from a narrative viewpoint (so, basically, the opposite of what happened to Trip in "TATV"), then I would be okay with them killing off anybody. For the maximum dramatic effect, it might help if it was one of the characters that actually got some decent screen-time, which is pretty much just Archer, T'Pol, and Trip. If the other characters had been developed better, then maybe Reed would be the best option (perhaps sacrificing himself in the line of duty, which would be fairly in character); Hoshi might work, if only for shock value; Phlox I think would be the oddball choice. And then there's Travis, of course, but would anyone really care?