I picked Vanguard. While Infiltrator trumps it in the first game, If you have the sniper rifle achievement already you can make the sniping your bonus skill and have it for the first game. In the second one you can acquire a sniper midway through, and in the third you have free weapon selection. Secondly, over the course of the three games, the class gets much better, which is a much better feeling of Shepard gaining experience than a class hat gets less powerful. Alternatively, you could go Soldier, Sentinel, then Vanguard through the 3 games to be the best in each. Finally, Vanguard is my personal favorite.