What is the best kind of flask? How do I know if it's "good"?

I want to get my fiance a flask for his coming birthday. I have a couple of questions first. where can I get a good flask and what makes one "good"? I don't mind spending a decent amount of money on it. i also want to get something engraved on it, such as his initials or our intials, but i dont know if getting both of our initals is appropriate - what do people usually engrave on them? is stainless steel better than silver? Any advice/guidance is appreciated.