Even though I've vowed to do a Premier Ball challenge for every Legendary I've met, it surprisingly isn't as tough as I'd imagined. o_O Albeit, it takes a few resets, but eventually the Premier Ball just manages to click its way to victory...
Critical captures always fail for me, ironically. ;_;
Anyway, with that being said, I think the toughest legendary one was Terrakion. It pretty much took the longest, but, to my recollection, I had a Fraxure or something, and False Swipe wasn't to my favor.
Otherwise, it'll have to be Volcarona. My Pokemon were severely underleveled when I met this beast, so I just relied on chucking Quick Balls and resetting when things got ugly (In other words, after the 2nd Quick Ball). It took over 10 retries, haha. xD