KOH
I've always had mixed feelings about Gear 5- On one hand, it's a fun form and kind of fits Luffy, but on the other I think it was kind of poorly written, it's clearly not something Oda planned in advance like some people say. He had Who's Who hint at it 27 chapters before the reveal and that was about it. If he built up the reveal a little more and also toned down the eye popping gag and what not, it'd be a very solid form, but it's clear he just didn't think of it or care about it until he realized he's done a G4 beatdown three times before and people were getting tired of it.
As for IJ, like Sir Yasheen said, it's a dope one but it's not much different from DJ outside of the considerable increase in power and speed. Though I still like it despite that.