HOW HAVE I NOT SEEN THIS THREAD BEFORE?! I won't give specific people since I really don't want this to come up in my next counseling session and I already get enough flak for spending "too much time in the holodeck", but here goes.
I'm willing to try almost anything once. The Humans/Bajoran/Trill look very similar. Aside from some spot and nose stuff.
Be careful with Betazoids. Sure, having sex with a mind reader might sound cool, right up until she senses you thinking about someone else. Not a pleasant conversation.
Klingons are a fun time on the holodeck, but make sure that the safeties are ON. Or you have quick access to Sickbay.
Vulcans, now there's a choice people overlook. Most of the time they're just going through the motions, but once every 7 years they go nuts. One of my favorite programs, "Vulcan Love Slave", is set during this time. Same warning as the Klingons, MAKE SURE SAFETIES ARE ON.
The less said about fucking a Naussican the better.
There's nothing wrong about waking up next to a Tellarite, but make sure you pick the right gender before accepting an offer for a tusk job.
Andorians, now there's a species I wouldn't mind spending a night with keeping her warm. Cold blooded my ass.
Deltans and Orions. Holy shit. The pheromones, man.
Finally, Saurians and Caitlains. Not like I'm going out of my way to screw a walking fish or cat, but like I said, I'm up for anything on the holodeck once. But research what 'coital hooks' are first.