TT Slade. Take away the helmet and it's a super generic outfit with nothing to distinguish itself as Deathstroke. The same could be said of the Arrow suits, but the difference is that they're trying to be somewhat realistic and are live action. In an animated show as grounded in whacky superhuman costumes as TT, Slade's is just too plain.
YJ ponytail gets on my nerves. The rest are pretty good IMO.