1) Singapore
incredibly safe, amazing night life, it is warm all year round, easy access to just about every country in asia as well as australia and new zealand, and the best public transport i have ever seen. singapore is just the ****.
2) ok i'm assuming visit and vacation are different in that visiting means up and walking around while vacationing means lazing around doing nothing to relax. so my favorite place to visit was probably greece:
amazing sites to see and friendly people. there is such a history there...one of the best trips i ever took.
3) vacation. hmm thailand. the beaches of phuket are amazing...even some smaller places like krabi are really nice. and amazing scuba diving.