Revelations was probably my favorite. The appeal of having an ace squad of rugged nobody's-turned-assassins, trained via generic mission menus and passive timers was somehow still engaging. It left me to imagine endless simultaneous 80s montages of slaughter across the whole of Europe with a team of assassins whose garments were inexplicably brightly colored. And though the execution of the game faltered a bit (pun, of course, intended), I think I'm in the very small group of people who actually enjoyed the tower defense mini game. I admit it wasn't good, but that didn't keep me from liking it. A lot. Also, obviously, seeing Ezio's story brought to conclusion was profoundly satisfying. Easy one of my favorite video game characters of all time.
As for Valhalla, I think I'm mostly excited by the medieval setting. Partially because I'm Scottish, but also because I think the modern UK is a woefully underexplored region, this being one of its most intriguing time periods. Roman ruins, Nordic "invaders", yet-unrealized empire potential as local feudal power struggles grow -- it's a gameworld ripe for interesting and historically-adjacent story-telling!