As VV said we do wifi tournaments but only for black and white.
But there are some requirements that your Pokemon team must be lv 100 and if you forgot to add Evs ect your screwed as 90% of people here basically hack the Pokemon into the games.
We won't do Hg Ss tournaments because there is b/w with more variety.
Showdown anyone can use considering your computer can take the lag, it's a website so if you have a good computer no problem.
Also you would not be able to hold the tournament, that usually is the job of high users like me.