I say ENT, due to the fact that all the familiar "alien" races are "new" to humanity, where you could really make a difference. Imagine being around BEFORE there was a "Federation" or "Starfleet", and technology is more familiar to the 21st century mind. In fact, I have a "dream" scenario where I would sign up for UESPA (after drowning my sorrows with a pint or two of ale). I would join up as a crewman, have adventures on board the NX-01 Enterprise (participating in the Earth-Romulan War), see the founding the of UFP, get caught in a time-warp that sends me forward (exposing me to temporal energies that retard my aging), re-enlist under Captain April, leave for Starfleet Academy, return as an officer under Captain Pike, then under Captain Kirk, get caught up in yet another time warp (which takes away the temporal energies that retard my aging), serve under Captain Picard, then transfer to DS9 (under then-Commander Sisko), get caught up in the Maquis (as a double agent), only to be whisked to the Delta Quadrant, where I would serve under Captain Janeway, share the last bit of temporal energy to allow Kes to age like a human, get caught up in an unstable wormhole with the Ferengi that takes me back to the Alpha Quadrant, participate in the Dominion War, discover a malfunctioning Iconanian Gateway that takes me back to the Delta Quadrant, continue to serve under Janeway again, return home, get invited to Riker and Troi's wedding, face off against Shinzon with the rest of the crew, assist Ambassador Spock in trying to stop the destruction of Romulus, only to end up in an alternate timeline where I would serve under a younger Captain Kirk...
I think that covered everything. ;-)