I will probably only see North American ones for a long while and I've yet to see Arctic Ring of Life. However, my top 3:
1.) North Carolina Zoo
2.) Columbus Zoo
3.) Louisville Zoo
North Carolina: This one is almost neck to neck with Columbus Zoo. However I chose it as top pick because there are two different areas the polar bear can roam and visitors to see it in. The main pool is big and there's two areas to view underwater. The new area is gorgeous, has different vantage points, and allow up close views of the polar bear since one area is a cave section. The graphics are always excellent and tell a story (Piper's Den).
Columbus: Beautifully landscaped, large pool, underwater viewing with a slight curve to the window so the polar bears can be above you. Half the front area is the pool so you can only see them really up close if they swim to you vs. all land area at the North Carolina Zoo. Cave areas, dirt to roll around in, a sniffing port.
Louisville: This is definitely not the most natural looking, but they have a deep pool (two stories tall) to swim in. Great viewing areas for guests and the rotational system allows different enrichment for the bears. The two younger bears, also a perk for visitors to see, seem so active.
Unfortunately most other ones that I have seen have been average. Some that may be noteworthy would be Maryland Zoo (for the tundra buggy in between both exhibits) and the Philadelphia Zoo (for being ahead of its time when built).