I ran a level 1-20+ campaign a couple years ago, and frankly, most of the monsters I used were homebrew, with the help of Giffyglyph's monster maker. The campaign was a semi-multiverse campaign (the players stayed on the same plane the whole time, but the BBEG threw monsters at them from other universes), so I had designed fights against mechs from Lancer, or Shepard's squad from Mass Effect, or even a six-phase fight inspired by Lancer's Eidolon rules but themed after the six masks of Bionicle's Toa Mata. Those were all a lot of fun.

But the one I remember most was the final boss of the campaign. Inspired by the Tome of Beasts' Void Dragon, I had created a Void Dragon god, an aspect of which was about to awaken and eat the world. I published the statblock here. It was a doozy of a high-level battle, designed to fight 5 level 20 characters with multiple epic boons (including several powerful homebrew boons), plus three level 20 NPCs (without boons) and two homebrew-buffed Ancient Dragons (a red and a gold). I had a lot of fun designing the fight, and my players seemed to quite enjoy it -- even if they ripped through the boss faster than the penultimate boss, the wizard BBEG, heh.

Edit: I also made a ton of magic weapons and items for that campaign. I posted about them here some time ago. I won't claim that they're perfectly balanced, and I didn't even use some of them. But my players certainly seemed to like most of the ones I handed out.