MNK is generally seen as the strongest melee. Tanky, shields can provide a surprising amount of teamplay (6k shields with 10 sec charge timers vs TBN being 8k on 15 sec... MNK shields are pretty good), and a guaranteed 36k damage hit that also stuns the target and breaks guard on a 60 sec charge is... kind of good? You should be seeing plenty of them. You tend to see SAM and DRG a lot in lower tiers, though, because they tend to be very effective at crushing people that don't know what they're doing. DRG is still effective at high skill, though limited in ways that other melee aren't. SAM... I dunno, you rarely see SAM in high skill unless they're just dedicated to the class. It can't really do anything that other classes aren't better at.