Queenly Majesty/Dazzling/Armor Tail: not sure why we need 3 versions of the exact same ability that are all signature to 1 Pokémon each, but blocking all priority moves is so good
Harvest: leads to fun battle shenanigans if played right. I once saw a YouTuber use Harvest Tropius with Liechi Berry in BDSP and it did a lot of work
Merciless: I know Toxapex is a bad candidate for this ability since it's not a very strong attacker, but I like to use it on Showdown on the Balanced Hackmons tier (any move or ability can be used on any Pokémon, with some exceptions) to give it to mons like Mega Gengar or Eternatus (with toxic spikes support) for a super strong Venoshock.
Skill Link: I guess Loaded Dice does something similar, but I like Skill Link + King's Rock. Diabolical strategy lol
Pixilate/Refrigerate/Aerilate/Galvanize: I hope we get more abilities like these for other types. Good for moves like Hyper Voice, Extreme Speed, and Return.
Sniper: I like to pair this with Focus Energy, Scope Lens, Night Slash, etc. for super-strong, guaranteed critical hits