I would go with the iPad Air 2 or iPad Mini 2 (not sure the 3 is worth the extra $100 cost).
Personally I would prefer an Android tablet over an iOS. Unfortunately I just haven't seen any really great Android tablets at the moment.
Nexus 9 was a big disappointment for me, it just feels too cheap and bulky.
The Samsung Tab S has a great display, but a very mediocre processor. Tab Pro is a mixed bag too.
Sony Z3 looks like a quality tablet, but I think over $400 for an 8 inch Snapdragon 801 and 1080p tablet seems a bit much, iPad Mini 2 looks like a much better deal at $300.
I still love my Nexus 7 though, especially with Android 5.0 on it. I think this guy is still a gem at $200 if you don't mind the 7 inch screen.