I'll chime in and give another nod to choosing your favorite system and rolling your own. I'm doing it for Thousand Suns right now, and I'll end up with the exact game I want.
But if time is an issue, then I agree that LUG's Star Trek -- especially if you're going to play Next Generation -- is really nice. Somewhat unusual mechanic and chargen, but serviceable. Moreso, IMHO, than Decipher. I really like LUG's TOS book, wish they'd been able to release more supplements for that version.
However if you are looking to play TOS, I'll say that as old as it is, the FASA system is a lot of fun and feels like Trek. I really love their original approach to bridge-command based starship combat (though Tactical Combat Simulator is nice too, it just feels like stepping outside the RPG).
Almost all Trek RPG stuff is oh-so-cheap on eBay right now, you can almost afford to buy all of the best.
Wouldn't touch Prime Directive (any version) with a 10 foot pole if I wanted to play Star Trek, but that's just me saying "it don't come in 2nd here!".