Don't really know anything about Serebii. It seems good in terms of news updates for pokemon, but aside from that, it doesn't seem any better than other sites.
Marriland sucks ass. Other than the people in Spirits Group (which I am a part of), everyone there is an arrogant prick who either complains about hacking when you beat them, pummels you with clauses, or who pull together gangs to bash you if you disagree with them.
I only go to Marriland to PM my friends.
Smogon is the absolute best for trading and strategy building, but it's networking capabilities suck.
As for this place, I think it's a bit comfortable because of the lack of seriousness among other members. It's not a constant struggle against bashing jerks.
It's pretty easy to make friends here. :)