The Murmillo has the coolest hat, but i'd put my money on Retiarius. Honorable mention to ol' Scissor, whose legacy persists to this very day in offices, classrooms, and first aid kits.
Yes for the Trax/Thraex. He has a small curved blade that can reach around an opponent's shield, and a smaller shield.
For the Murmillo and Secutor, it's a hat competition. Secutors were usually pitted against Reitiarii - the small eyeholes in the helmet were to keep the trident, specifically, out, and the helmet had fewer features to make it harder to net.
I dig the Murmillo more, personally. The helmet has, as the Romans would say, gravitas!