My vote goes to Alex Lifeson from Rush. Geddy and Neal were the icing to his cake. They had a great run, but in the end, Geddy couldn't sing and Neal couldn't play like the athlete aliens they were at 25. But Alex kept right on playing fantastically. I might even argue he got better in the later albums. I can't say much for the song writing then, but the playing was fantastic and I say he held that band together for the past 15 years.
I also have to give it up to Johnny Marr. I thought he was the best thing about the Smiths, and that's saying a lot.
Back in the day...it's kind of insulting to call them 'sidemen', but Nugent's original band - Davis, Grange, St Holmes. I met St Holmes at a work thing and lost my mind, even after so many years later.