On level 50, it doesn't really matter which tank is MT. I could imagine that it would be PLD or DRK. DRK bc they have the highest self-heal, PLD bc they have Sheltron at this level.

The reason as why you couldn't get aggro from a GNB is most likely your gear. You are new, and Lodestone says you still have an NQ lvl 46 weapon. Whereas GNB is a job that starts at 60, so their weapon gets synced to max stats. Additionally, GNB probably does the most dps as tank, but I can't say for sure, it's just a hunch.

Last but not least, you shouldn't fight over aggro with your co-tank. Unless there are mechanics happening that force you tank-swap -which again don't ever happen in normal or hard mode content, but only in extreme and savage content- (correct me if I am wrong) you never fight over aggro bc this will just irritate the party and the flow of the fight. Turn off your Iron Will (tank stance), and fight along the dps. Oh, and stay behind the boss, no reason to take extra boss cleave hits from the front.