Peter Criss is definitely the lucky member of KISS, and by the point they'd reunited there, the only appeal was that he was an original member. There are worse clips of Criss from the reunion tour, but even at his best, he had no power or energy left, and he thinks much more highly of himself than he ought to.
The rest of the band members however, I'd argue were crucial to the success of the band.
Paul Stanley was the key songwriter, and an underrated singer and frontman in general. He is the main voice of KISS, and the band could not have survived without him, especially during the '80s when they relied on him more than ever.
Gene Simmons isn't the best singer or bass player, but his bass lines have always been memorable and melodic, and he also contributed to songwriting and vocals, although he was never as strong a songwriter as Paul. He's also perhaps the most recognizable and iconic member of the band, and sang many signature songs, and I think he was critical to the business side of the band and getting them off the ground too.
Ace Frehley I'd say was also an important factor. Again, not the flashiest player, and not one of the best of the era, but he had such a distinctive style that became a key part of the band's sound, and was a big influence on many later guitarists. Maybe they would have gotten by with another guitarist, but it would have changed the band drastically.
Peter Criss was a competent drummer back in the day, but I think you could have put any other decent drummer in his place, and not have lost much at all. He had a nice voice, and sang some good songs, but I don't think any of his contributions were instrumental to their success. He was lucky back in the day, and downright blessed to be brought back in the '90s and later.