If arenas is there GET HIM. no questions. Unless someone Kobe/Dirk/Marion/Yao somehow falls to you. I would say chris paul, nash, allen iverson, wade, bosh and paul pierce are all safe bets, although if it were me I'd probably go with wade or bosh just because wade seems to be statistically the best amongst those people and bosh qualifies at C.
I would not go for Amare or O'neal at 15th. If you need a C, those two might drop to you in the 3rd round, and you could always pick up Duncan, Camby, Okur or Z in the 3-5th rounds. They are all good 1st Cs.
T-mac is a tough choice, his back seems ok, so 15th should be a safe bet for him, but it's a risk that personally I would not take.
His team is so good that they coast through the regular season and he sits out games. And in some games that he actually plays in, he either doesn't play many minutes or just lets everyone else do their thing.
Howard will be among the league leaders in FG%. His FT% will suck ass, but Arenas' will be good and he doesn't shoot that bad from the floor.