If you're one of those whose bought Assassin's Creed: Unity's DLC Season Pass at launch, you're getting a big fat Sorry from Ubisoft whether you felt you needed it or not. They need your forgiveness. They burn for it. The publisher's apologies for Unity's many and various bugs and performance issues comprises a choice of one of the following: Far Cry 4, Assassin's Creed: Black Flag, The Crew, Rayman Legends, Watch_Dogs or Just Dance 2015. We already knew that! But now it's actually happening right now go go go go
...Or try to. As is something of a Uplay tradition, the system promptly fell over when first turned on, though it does appear to be working now. Sadly you're stuck with whatever format you bought ACU on - I wouldn't have minded adding something new to my little-used Destinystation. Also Just Dance is the only game other than The Crew that I don't already have, and that's not available on PC. Boo!
Anyway, clearly you should choose either Far Cry 4 or Black Flag, or Rayman Legends if you already have them. I went with The Crew purely cos I didn't already have it, but I'll wait to see if it has any major updates before I bother.
If you want your free game and somehow missed the email, the link is here. I'm pretty sure you can't pick up a skeezy ACU key from somewhere to get a free game retroactively, though. Once you've picked, you'll get mailed a Uplay code. No Steams, I'm afraid.
You've got until March 2015 to choose, so maybe wait and see what Santa brings before you commit, eh?