I don't really know if I have any "best" endings.
But worst:
-- Adventure Express. I love the rest of the ride.. and the lift hill itself is the best lift hill EVAR~! but.. the finale is just.. what the hell!
-- Vortex at PCW [Top Gun at PKI]'s super swing into horrible brakes. Ughhhh.
-- Great Bear's entire course was pretty lackluster [made worse because I had high hopes for it], but the ending was especially lame.
-- Volcano's "Let's drop down this huge hill and get wicked speed and then OOPS YOU'RE DONE" ending really, really sucked.
-- Storm Runner's ending was abrupt, way too early. I guess it sucked, just because it put an end to a ride that was shaping up to be excellent.