I have to agree with Kurgen about France- a wonderful place to visit with particularly lovely girls.
I did my year abroad in Singapore because I was interested in that part of the world. Got to travel a lot, met lots of people, fell madly in love, dived, learned Tae Kwon Do, had a generally nice time. I also think the night life has improved dramatically since I was there. Also if you like small butts, Asian chicks. ALso its tropical.
I would stay away from Australia unless you are going to the coast. Great for diving and sports and I like Australians, but if you are going to Canberra, well, its in the middle of nowhere.
US- go to New York, Massachusetts or California. Avoid just about everything else. NO offense meant to any other poster here, but if you are coming from abroad to go to a country you want the most bang for your buck.
Lets see, Japan is neat if you are into the Japanese. I would avoid China and Russia.
Thais are great folks, but Bangkok takes a bit of getting used to. Polution is heavy and I think they still have flooding problems. Great place to see and visit and a good location to travel from. Otherwise, if you want to go to Southeast Asia- SIngapore is better.
You might also consider South Africa (unless you already have the "been to Africa, done that" feel.
Canada- no. SOuth of France is much better. Go some place warm.
Argentina and Brazil- Brazil you have to be careful. Both major cities have lots and lots of crime but are lovely places otherwise. Argentina is in economic trouble- better to avoid it.
There is always Spain. Barcelona? Madrid?