The Secret World is not my game of choice, but I sure did like its system for appearances. Clothing was just… well, clothing. You could wear whatever outfit you liked! Your stats were entirely tied to other parts of your character, and there was lots of stuff to mix-and-match. Skyforge uses a similar trick with its outfits, but there’s less mixing and matching. It’s a bit less appealing.
Of course, I’m very fond of any cosmetic system; there are just better and worse options. City of Heroes has some gold-standard elements to its cosmetic system, naturally, but even in games with more investment in gear, there are better and worse systems. So what MMO has your favorite cosmetic equipment system? Would it work in other games, or not quite so much?