Best acting by the main characters? Kira as the prostitute in "The Homecoming" and the gangster's moll in "Badda-Bing, Badda-Bing", or Picard as a smuggler in "Gambit". As for the worst, it has to be Kirk as a dying man in "The Enterprise Incident", it's a mystery how the Romulans bought it...
Oh... you meant, best and worst acting by actors who play main characters?
OK, then:
Best:
TOS: Leonard Nimoy
TNG: Patrick Stewart and Brent Spiner
DS9: Nana Visitor, Rene Auberjonois and Colm Meaney
VOY: I'm just starting with VOY (still in season 1), so I can't be sure; so far, I suppose Jennifer Lien or Tim Russ
ENT: I don't know, I haven't watched it past the pilot
Worst:
TOS: if you mean just the 3 actual main characters, then obviously it has to be William Shatner; if we count "The Cage", Jeffrey Hunter; if we stretch the definition of "main character" to the supporting ones, then of course it's Walter Koenig (worst acting out of all regular characters)
TNG: Will Wheaton or Denise Crosby
DS9: Alexander Siddig, at least early on, gets better later (but OMG, he was still awful in those MU episodes)
VOY: Ethan Phillips without any doubt. He ever ruined his showcase episode ("Jetrel"), which is sad since the story and the guest star were really good.
ENT: I don't know (see above).
)
