Gonna have to go with trifecta of TNG women... Gates McFadden, Denise Crosby and Marina Sirtis. They were all abysmal during TNGs TV run. It really wasn't until the films that Sirtis appeared to find Troi, and as long as she was in a slightly comedic scene she worked just fine.
Unfortunately, neither Gates nor Denise is a terribly good actress. Gates is extremely limited as an actress and I found Dr. Crusher to be one of the worst characters in all of Trek. Denise was just.... well, interesting. In one episode she could turn in a serviceable performance... but just re-watch "Encounter At Farpoint," specifically when the crew ends up in Q's "courtroom." Her overblown, over hysterical performance when she is lecturing Q about the goodness of the Federation always elicits a laugh... we are talking MST3K territory here.
Beltran gets a special nod for just not caring and showing up for a paycheck.