Surviving Mars is free at Epic Games until tomorrow. The next week (Oct. 17-24) Alan Wakes American Nightmare and Observer are free.

(I enjoyed Alan Wake when it came out)   

That reminds I got the free Batman Arkham trilogy from Epic a few weeks ago and after registering, I had to do a password reset, but they never sent me the email until it had already expired. Anyway I finallly got that sorted out... since I'm not going to install all 3, can anyone recommend which is the best -- Knight / City / GOTY ?

Arkham City. Knight is good, but has too much (sometimes ridiculously difficult) emphasis on Batmobile missions.

My favourite, which is not on your list is still Asylum.

Arkham GOTY is the same as the original + DLC. Never bothered with either.

Thanks

The City game is the best of the lot, but the first one walks you through the mechanics that evolve throughout all the games and can feel a little basic if you play it after the sequels. 

I'd say the story in Knight tops the story in City, but both are equally far fetched and stretched out (given the playtime on these games is between 20 and 30 hours). 

Still, I think they're all worth playing.