I prefer the Emerald frontier. Along with the facilities in general I also prefer the designs of the Hoenn Frontier Brains to the Sinnoh ones.
My favorite facility is probably the Battle Dome, and although they could be frustrating at times, I also enjoyed the exploration aspects of the Battle Pike and Battle Pyramid. It was pretty neat that they weren't just straightforward battling or battling with a gimmick like the other facilities (not that there's anything wrong with those, it was just cool to see).
My favorite Frontier Brain is Anabel, with Lucy as a close second. Lucy used to be my favorite until Anabel showed up in SM with a new design and an interesting role involving the Ultra Beasts.
I'd definitely like to see the Battle Frontier return in the future, but I'm not sure how likely it is since the reason given for it not returning in ORAS was that they thought since kids go from game to game pretty quickly, they wouldn't spend much time doing the Battle Frontier.