I think the green to southeastern boaters is the same as the big south is to colorado boaters.
A green boater can run the big south and say "so what, I've run better stuff" and a big south/colorado boater can run the green and say "so what, I've run better stuff" so the comparison is hard to make.
In the southeast - the green is definitely the river of choice. There are many other rivers here of much higher quality (raven fork, linville, toxaway, etc) but the people that run those rivers probably run the green at least 10x for every single run on one of those harder creeks. It's easy for us to take the green for granted, because it's always running (been always running high lately), but when everything is dried up around the se in the summer - it's a mecca.
My favorite run in colorado is pandora's box, followed by the vallecito, followed by obj. The Clark's Fork was pretty rediculously awesome - but A LOT of effort for a handful of good rapids - still worth it though - just make sure you bring enough water (I didn't).
If you're ever in the southeast and you want to run something a little better than the green, check out overflow creek in the chattooga drainage - it's twice as long and has much better/bigger rapids and it's probably easier than running all the rapids on the green, but a little harder than green light. Also, if you want to get scared, make sure you check out the raven fork and toxaway.
In the costa rica conversation - my favorite there is the pozo azul, but then again its the only big run i did while I was there.
As for the rest of the world, rio palguin in chile looks awesome.
This is a good thread - I'm looking forward to seeing what others have to say.
Kevin