Honestly, I always liked the series best. Perhaps I'm just weird, but I've always felt the original Highlander was kind of meh. Not a bad movie, by any means, but not the epic that some people feel it to be. YMMV.
The series did the concept justice once it hit maturity TNG-style. The original movie, while pretty good, was plagued with too much....um....80's. It looks like a music video in the modern sequences and the style was way too over the top.