Depends.
Out and about with it all the time, you might like the extra portability of iPad Mini Retinal better.
Mostly at home or sitting down, the bigger screen of the iPad Air will make both video and games more enjoyable.
I don't recommend buying an original iPad Mini or the iPad 2 at this time; for various reasons. Not unless you have a specific use for them that does not require updating software.
If you can get a deal on an iPad 3 or better yet a 4, you'll should be happy with it for at least a year, and find it usable for a year past that; but you'll probably end up wishing you'd sprung for one of the new models.