How can it not be Q? He / it makes it personal and John Delancie brings a mountain of personality to the role. The simmering disdain with just a hint of condescending encouragement. The godlike power behind carefully cultivated aloofness that conceals his genuine affection for Jean Luc Picard. He seems to carry all of that effortlessly.
Nobody else comes close on my opinion.
Lower Decks made the Pakleds compelling antagonists by leaning into their already canon ability to use their appearance of simpleness to manipulate the larger powers. I never expected that.
Romulans are always nice as a dark mirror to what the Vulcans would be if they didn't turn logic into a religion.
I thought Klingons were boring until DS9 fleshed them out. Still not a "favorite" antagonist.
I feel like they dove into the Borg well a bit too often. I also played a ton of Star Trek Online where the early years built-up to the Borg returning and the first repeatable Task Force Operations were all Borg for too long. So that probably is another reason I got burnt out on the Borg. The game eventually brought in the Iconians and did an amazing job. If we were talking about STO, I'd put the Iconians at the top of the list.