By far the easiest facility is the Arcade. With a quick enough reflex - and you should have one if you're playing video games, right? - you can give your opponent a natural disadvantage at the start of the fight, giving yourself smooth sailing for pretty much every battle you encounter. Castle would probably be the next easiest, just so long as you know to give all your opponents level-55 teams for the first 15 battles or so (then you can upgrade your healing and items very quickly).
Hall is a lot harder than it sounds. You ever try going all the way to 170 using just one species of Pokemon? I mean sure, you can use two different ones with two different movesets, but it's INCREDIBLY hellish.