The original. It's a film of two halves; I love the flashback material, which is pretty timeless, and the then-present day stuff has dated horribly but is perfect 80s-cheese fun. Christopher Lambert is about as Scottish as a croissant, but it kind of works - he's got the rhythm and the cadence down, just not the vowels - if you consider that he'd probably be speaking a medieval Gaelic dialect anyway so it hardly matters! At least in the present they address it as him having been all over the world in 400 years. That said, Lambert as a Scotsman with a French accent, explaining haggis to Sean Connery, playing an Egyptian with a Scottish accent is one of the most bizarre moments in film history.
I'm a fan of the TV series so I found the fourth film enjoyable despite its flaws - the main one for me being that Lambert is clearly 30 years too old to still be playing an immortal who's supposed to look 18 (albeit an 18-year-old of the year 1536 - he was 28 when he made the first film). It was at least fun to unite Connor and Duncan, even if it was the Connor of the series' continuity; continuity being a word to be used lightly in relation to Highlander!