Although I still feel that the original Highlander should be considered a stand-alone movie, if you really want more, then Highlander III is worth watching.
Thankfully, this movie ignores the woefully mis-guided Highlander II as though it never existed, and continues 8 years after the events of the original.
While the plot is essentially the same as the original, it does have some highlights, such as revealing more of Connor MacLeod's past after leaving the Scottish highlands and a nice musical score involving celtic themes and the song 'Bonny Portmore' by Loreena McKennitt.
Like the original, the low budget special effects and sometimes poor acting are a downer, but if you can see past that, then it's not all that bad.