So i'm just gonna go with "at least equally as good as their best material" as my definition here.
It's far from their best song, but I love Pink Floyd's "High Hopes". People give the Gilmour era a lot of shit, but they could still do good stuff.
Some people seem to forget that Yes broke up competitively after Drama meaning that 90215 and everything that follows qualifies for this poll. top 5 post-reunion Yes songs (off the top of my head and unordered): Changes, Leave It, Endless Dream, Magnification, Fly From Here
Phish's reunion album Joy is one of my favourite studio albums in their discography. I'm a fan of just about every track on there. They've had a very fruitful reunion. Top 5: Backwards Down The Number Line, Mercury, Petrichor, Blaze On, Steam
Post reunion KC top 5: Elephant Talk, Frame By Frame, Sleepless, Level Five, Dangerous Curves
The Monkees "Good Times" is maybe the most solid-back-to-front album they ever did. So maybe that could count for this poll! Did they ever actually breakup though?
Guided By Voices probably have some qualifiers, but I've long since lost track of their reunion albums.
Does everything Weezer did after Pinkerton count as reunion albums? If so then they have plenty of candidates. "(Girl We Got A) Good Thing" maybe is my favourite.