Tonal Island is about the best looking island out there. It is a rather large download, unfortunately, and it can get somewhat laggy. Don't have a link, but it should be easy to find. It is an African setting.
Dak Seang island is about the most amazing island I've ever seen, aside from Tonal. It is a SE Asian setting (Vietnam), is only about 30mb, and only requires 1 addon (a bridge pack).
The problem with Dak Seang, however, is that the AI can't move thru most of the forests on the island. Pretty sad that someone would release such a great looking island, but make it almost useless for actual missions. It is a new release though, so hopefully it will be fixed in the near future.
There are a few other Vietnam maps that have been released recently, but most of them require a fair amount of other addons to run (A Shu valley and some others). Actually, most seem to require Tonal island.
So again, either Tonal island, or Dak Seang are the best looking, IMO. There are quite a few islands using the SEBnam objects which are better than Ia Drang, although I don't like the SEBnam stuff. Head over to OFPNAM for more:
http://www.ofpnam.com/news.php