There are a set of mons In the Pokemon meta game that define the meta of OU and Ubers and even create a new tier. If you’re creating a team in these tiers, these mons might be a good start
Snorlax: An extremely decent physical sweeper with good bulk. Thanks to the move curse, (a move witch lowers speed but raises attack and defense) Snorlax can become a terrifing threat with body slam, Rest (witch puts the user to sleep but fully heals you) and earthquake. In Gen 1 it dominated the OU meta game and is still an effective mon in OU.
Landorus-Therian: a terrifing mon in OU with 125 attack, blazing speed, excellent typing and access to swords dance witch doubles attack and earthquake, and Ice punch. Not much can stand in the way of this terrifying mon capable of OHKOing (one hit knock out) most non bulky mons and outspend most mons in the tier
Gardevoir-Mega: Certain mons in Gen 6, have been given a mega evolution. a newer, more powerful, form that lasts for the entire battle. Gardevoir is one of these mons. When it mega evolves, it gains even more special attack and speed than its already impressive stats and gains the ability Pixilate. witch turns normal type attacks into fairy. Gardevoir is already fairy witch makes these normal turned fairy attacks STAB (Same Type Attack Bonus) and gain 1.3x the damage. this combined with the normal type attack Hyper Voice make this mon terrifying to anything that isn’t Fire, steel or poison type (witch all resist fairy) but thanks to Gardevoir’s secondary type, psychic, super effective poison types and hit fire at least neutral with a STAB psychic make it a terrifying mon to face.
Rayquaza-Mega: This mon literally created a new tier: AG (anything goes). With 120 in each defense, blazing speed, the ability to hold an item and mega and balanced attack stats. nothing much needs to be said about this terrifying mon