I would steer clear of Enterprise in general because the show really hinges on familiarity with TOS and/or TNG, but i would doubly stay away from Cogenitor. If that was my first Trek episode, it certainly would have been my last

I'd echo hayvan that TOS "Trouble with Tribbles" is a safe bet, very silly on the surface but has all the trappings of good Trek. if that (or TOS in general) is too cheesy, SNW "Ad Astra Per Aspera" is more modern and more serious episode that's still very representative of Trek as a whole