To be 100% honest with you, your first class all the way to 50 should probably be a healer or a DPS, as these roles require a lot less knowledge of the game to perform well. Now if you are a MMO veteran, and comfortable with the responsibilities of tanking, then more power to you! But I typically recommend DPS classes for peoples first 50's.

And both MNK and DRG are great classes. I prefer DRG mostly for aesthetic reasons, but I WILL be levelling my monk soon, since I love the design of the class.