Pokemon deemed too powerful for the well-being of the metagame are called Ubers and dumped into a tier of their own.
Randoms and brs tend to ignore this due to their general lack of knowledge of competitive play (or anything) and consider them prime choices due to their sheer power and high stats. It's common to see newbies and noobs with over four ubers.
However, the uber metagame is still there to give everyone a chance to use these Pokemon.
Whenever a player has one uber or more on their cloaked team, a red message will apear on their challenge window saying "[number of ubers on team] ubers."
List of RSE ubers
- Deoxys (all forms)
Note: Celebi is considered uber in GSC.
Ubers and Legends
Noobs often mistake legends for ubers due to the fact that both are 1-shot Pokemon in the RPGs, so it's a common thing to have randoms accuse you to use ubers when you send out Celebi, Zapdos and the likes. Ubers have base stats over 600, legends don't (although Celebi's tier in RSE was questioned a few times.)