Users are usually Pokemon with really high Base stat. Usually offensive, defensive, or supporters. The majority are legendary, excluding wobbufet. Offensive Pokemon like Rayquaza have high atk or s.atk can and can set up and sweep. Pokemon like Lugia have high defensive and hp stats. They can wall Pokemon and stall them out with hp recovering moves. Supporter Pokemon like mew can set up and help other Pokemon sweep.
Here's a link for all the Ubers for 4th gen