A good player can take any tank in this game and excel with it. The classes are all pretty solid. Stat counting and min maxing are fun. No doubt, but there is no situation where a particular tank just can't do the job (Like we had in wow where only DKs could do Vezax because they were the only tank with applicable cooldown CDs).
You can excel with any tank in this game, but the classes are not as solid as they seem. Guardians, especially due to faulty defense tree have big problems holding threat and this denies the dps their full rotation execution the first 30 seconds of the fight resulting in a significant dps loss or loss of aggro. This was not meant in any way to happen by Bioware, and I think with new level cap they'll be taking measures to fixing this.