I'm using the Gibus although I own alot of other hats. It's just stereotypical people that always believe a hat means skill so it's pretty fun wearing it along some pyrovision goggles and cause some mayhem as "that F2P". Plus you don't get alot of heals, proven by my endless studies of the Gibus Arts.
Now that we all without any doubt agree that the Gibus is the most underrated treasure in your backpack and that #givegibusachance is trending #1 worldwide on twitter we can come to the worst hat in the game.
The worst hat(s) have to be any with an unusual effect, since these will just let people know where you are, or let them recognize you over and over again. People with unusual hats usually also have that crappy personality of thinking they are above f2p or anyone else. Just because they own some pixel. In a game. Like. Really? Oops, am I breaking the 4th wall? Am I hurting you? Sorry.
Now that we established that unusual hats mean = totally visible to enemy, recognizable + corrupt your personality and social behaviour we can establish and come to the all universal conclusion of what was mentioned earlier: Use. The. Gibus. If you need to have something fancy, strangify it.
The Gibus is life. The Gibus is art. The Gibus is freedom. The Gibus is an epiphany.
Give it a try. Don't jump at a hat you never wear ;)