Dark and Ghost Types
Used for the same reason, they are super effective on Psychic types. There are tons of Psychic types in Ubers so these guys are very powerful. These guys get used a lot for their super-effective STAB.
Steel Type
Used because of all those resistances and immunity to Toxic. Very hard to defeat and like most Arceus can be a pain to defeat after using a couple of Swords-dances. Arceus can carry many stalling moves like Recover, Cosmic-power, Toxic, Wil-o-wisp, and so much more. Definitely hard to counter.
Water Type
Usually used in the rain. So Judgement+Water-plate+STAB+Rain = VERY POWERFUL! Usually also carries Thunder for obvious reasons. This can be a pain to deal with. The best way to deal with it is changing the weather.
Fire Type
Similar to the water type it is paired with Sun. So Judgement+Flame-plate+STAB+Sun = VERY POWERFUL! Fire has better coverage than water but sun the fact that there are so many rain teams makes it a little harder for this one to shine. Changing the weather is devastating for him.
Normal form is also used often, because you can use any item you want and have STAB Extreme speed. Usually it is safe to guess that two of its moves will be Swords-dance and Extreme-speed.
Honorable mentions: Electric, Ground, and Fighting
The least common ones are probably: Grass, Psychic, Poison, and Bug do too either common weaknesses or lack of STAB coverage.
