Keep in mind that the amount and quality of the reflective material makes a BIG difference. A cheap $5 reflective vest won't be as visible at night as the one made with better retroreflective material.

Personally, I use at least an ANSI Class II vest from Home Depot or Lowe's or whatever. It's usually around $15-20, and has a large surface area of high-quality (usually 3M) reflective tape.