There's a few variation sets I've been enjoying as of late (mostly Slavic in origin), here are some of them:
Vasyl Barvinsky - Variations for piano sextet:
Vissarion Shebalin - Piano Trio, mvt. 3:
Anton Arensky - Variations On a Theme by Tchaikovsky (from String Quartet no. 2, mvt. 2):
Boris Lyatoshynsky - Piano Trio no. 2, mvt. 4:
I also like the Beethoven C minor variations, they are probably the most quintessential of all variations. But these are just some I think are also really great, and really creative.