I kinda like Gear 3rd a lot, because of the versatile uses of it. If it was just Gigant Pistol and Gigant Axe, I would find it boring. But I really like its combination with the other gears. Jet Shell (Gear 3rd + Gear 2nd), which defeated Moria, was pretty epic in my opinion. Tankman (Gear 3rd + Gear 4th) and Gigant (Gear 3rd + Gear 5th) also contribute to making the other gears even more interesting. So I wouldn't dislike Gear 3rd given its useful applications.
I think Nightmare Luffy was my least favourite version of Luffy. After absorbing ONE shadow, he became an expert swordsman. Thus, after absorbing a hundred shadows, I thought he was gonna pull awesome unknown techniques of skills. But all he did was punch. He was being himself, which is great but totally unoriginal. He just dealt one slash to Odz with his big sword at some point, but that's the only non-Luffy thing he did. Plus, Odz was still standing after fighting Nightmare Luffy: didn't any of the hundred souls have any notion of any martial art that breaks the opponent's joints and prevents them from ever standing again? Nightmare Luffy was extremely underexploited in my opinion.