It depends on if you are talking about pure fighting skills and if you count upgrades that overpower the user.
Eiji cannot be considered bad as he never really got any super form. He may have been beaten quite a lot of times, but he was never powered up more than usual by a specific form, other than PuToTyra, which even that wasn't really incredibly overpowered. He was just making sure to use the medals that could give him an advantage. So he did a really good job at fighting, even when the greeds were in complete form.
Kouta is the total opposite. He relied way too much on his power, and at points he totally failed. You can't consider him a terrible fighter, though. He went on even after he got a severe beating from Zangetsu. He tried and tried even when there was not much he could do. He had the determination of a good fighter, but still couldn't do much. If Michi had that determination, he could have beaten him without the need of Yomotsuheguri, just like when he did it with Melon Energy Arms.
Anyway, I guess the one and most notably one is Ryoutarou. Without an Imagin, he could do nothing, with an Imagin he still did nothing. When he got the Liner Form, he was still that useless guy we met on the first episode that was so scared to pass a ticker above a belt. You'd guess that with all the possession he got, he would at least learn a couple of stuff, but nope, nothing... Even his grandchild is a better fighter...