The three biggest decks I have identified as pains in the rear end are Cloudpost, Infect, and Affinity. Cloudpost requires any deck you run to be aggressive enough to get around counterspells and spot removal or resilient enough to not care about those things. Infect can and will make almost any deck play as the control deck, even if it's normally highly aggressive; the potential for a turn two blowout is not ignorable and a turn three blowout is almost certain if you do not have any answers or defense by that point. Affinity is downright unfair; you're going to have to compete with free or nearly free 2/2s and 4/4s, 2CMC 4/4s, burn for 4, pings for 1, 10/11s bashing in, 10/11s being flung in, 2 cards being drawn for a U, a 3/2 flyer for a U, and so on. In short, insanity.
Any deck you make has to be able to consistently win against at least two of these three decks if you want it to have a decent chance in any given tournament. If you manage to make a deck that can consistently win against all three, good job cheating.
Also, as a final note, it's a good idea to use monored burn as a baseline test for you deck; monored burn is so cheap and easy to play that it gluts tournaments and can consistently kill you by turn 5 or 6 if you do not have life gain or kill it by then. If you make an aggro deck, it has to win against an empty board before turn 5 while on the draw. If you make a control deck, it has to either have lifegain or a profitable way to counter burn. Burn decks are just too prevalent to not have game against them.
Thanks for the great advice. I'll be testing against those decks suggested.
I'm running WW Hawks and WW Soldiers. I'm pretty consistent vs Post decks b/c I win the race. Not as good vs affinity b/c of the cheap critters. I do fairly well vs infect b/c I've include some cheap first strike dudes in there. I'll be posting some decklists soon.