Pokémon Showdown allows you to engage in online Pokémon battles with other players. It only requires a web browser and works on mobile devices. You have the option of playing random battles with random Pokémon or creating your own teams. It is fully animated and includes all Pokémon from every generation.

You do not have to complete your registration before you begin playing. It is quick and easy to start playing, simply enter a username and click "Battle" to begin battling with random Pokémon against a random player. If you decide to use the teambuilder to create custom teams, you must sign up in order to save your custom teams.

You can chat with the other player while battling, which is great for making new friends. Battle replays can also be saved. If you don't want to play or want to see other players' strategies, you can watch live battles and even chat with the players as they battle each other.

Teambuilder

The Teambuilder lets you customize held items, movesets, stats, level, nature, ability, and more. It is possible to select whether or not the Pokémon are shiny. You could recreate the team you used when you first started playing, or you could create a strong team suitable for competitive matches. You can even form a team of shiny legendary Pokémon if you want.

The Pokémon in the team below are those I used when playing Pokémon Omega Ruby and Pokémon Moon.

Battles

There are numerous battle formats from various generations, some of which I am unfamiliar with. You can create teams that meet the requirements for each of these battles using the Teambuilder. You may be matched with high-ranking players at first, but as you lose or win, your rating adjusts and you will be matched more fairly. I got wrecked a few times but began to improve after a few battles.

Best Pokémon Battle Simulator

If you've ever played Pokémon, you'll love the ability to play multiplayer battles here. The website is still in beta, but it is not lacking in features. With the option to select Generations, anyone who has ever played Pokémon will be able to quickly and easily revisit competitive Pokémon battling using only an internet browser.