Worst actor without any doubt is AVERY BROOKS.
He is the breathiest actor on the planet. He huffs and puffs like a locomotive.
Terry Farrel was also a horrible actress! Her whole idea of playing someone "wise" and "old" like the Dax trill was to be really really quiet (tip-toe, tip-toe) and hold her hands behind her back. AWFUL!
Kate Mulgrew was actually a really good actress, but her voice takes getting used to. I disagree with the poster who criticized Tim Russ, I thought Tim did a fantastic job of emulating Nimoy. If you actually watch DS9, Michael Dorn's acting somehow managed to get worse?
On TNG he had short one-liners, and I guess those suited him. On DS9, his delivery is off by miles in some cases and he's just not the same character. His voice is also not as deep... Strange.
The best acting by leaps and bounds is Patrick Stewart, it's obvious he's a Shakesperean actor. I also enjoyed most actors who played the Cardassians, including Marc Alamo. I thought one really well-performed and memorable role was the Cardassian who interrogates Picard in that TNG episode. His "Nazism" is spot-on.