(This was originally a comment on the
32Bit Cafe forum, but I thought it may be useful to others, so I'm posting it here, too.)
There are a few different Pokemon forums to choose from; all of which being extremely active.
Bulbagarden is my personal go-to. It’s the same service that hosts Bulbapedia, the independently-run Pokemon wiki which any Pokemon fan is sure to be familiar with, as well as Pokemon archives and a podcast. It has almost 31 different forum styles to choose from, including a queer pride-themed one, which is a good sign in terms of political leanings. (Gotta be careful with gaming forums...) There's also a subforum dedicated to shipping, which is useful for fannish purposes, and which no other forum here has.
Serebii also has an affiliated forum. While Bulbapedia serves as a wiki, Serebii is mainly known for its guides, databases, and news. In particular, it is known for PokeEarth: a complete listing of all in-game Trainers and catchable Pokemon in every city and route. However, I’ve heard bad things about the forums going down a lot, as well as a reputation for malicious/trolling elitist users. It has only 2 forum styles to choose from, too; neither of which I find aesthetically appealing. This is not at all a knock on the collective; Serebii's main site is an invaluable resource that I personally use regularly. But it's maybe not your best bet for a forum.
Pokecommunity is not affiliated with any Pokemon resource, like the previous two, but I have heard of it through good word-of-mouth alone, making it a sort of underdog of Pokemon forums. The community is extremely friendly, and it’s easy to find other users to battle/trade there. It is also the only forum listed here which hosts subforums for fangames and hacking, so, if that’s your scene, it’s definitely your best bet. It has 11 different forum styles.