The best online battle simulator not just for pokemon, but for games period. Not just video games. The UI is perfect, the process of making accounts is super simple, the gameplay is super streamlined, spectating is a breeze, new pokemon from new games/DLC are typically added within under 24 hours of realese, matchmaking and sending challenges are implemented extremely intuitively, and, perhaps most crucially, you can take teams that you'd normally need to spend dozens of hours making on cartridge and spend hundreds of dollars on all the games for, and go ahead and make the team you wanna build in less than 5 minutes on here. You can even try using sample teams with a process that is literally just copy and paste- which is excellent for helping you get into the dozens of metagames you can play on here, and making it easier than you could possibly imagine to find a favorite way to play. There's a reason that even VGC players use showdown's teambuilder when making videos, testing teams on ladder, or sharing them with the world- showdown is simply the PERFECT tool for competitive pokemon.

THAT BEING SAID, this website will ruin your life. Do not be tempted. Competitive pokemon is genuinely fucking terrible, you will never be good at it, and you should find a better hobby before wasting years of your life chasing improvement that will never come.