When Highlander arrived on TV, it started out with a good concept (taken from the first film, primarily), a real hero (Duncan - played with great self-assurance by Adrian Paul), but some bumps in the road that gave it a TV feel. This is evident in the first season, as the episodes are hit and miss, and the show wasn't operating on all cylinders until near the end of the season. In the 2nd year, Highlander hit full stride, and with each successive episode, the series became more compelling. Many episodes from that year are excellent. The full Highlander team had to have known they had a winner, and the creative energy is high. These were the shows that solidified the fan base the show would ultimately have.
The 3rd season picked up right where the second took off, and by now Highlander was thriving, if in relative obscurity. By now Adrian Paul WAS Duncan MacLeod - The Highlander. The show had a level of synchronicity and often intensity, that made the very most of the concept. The actors (again, espeically Paul) the writers, and entire crew were obviously very inspired, as they turned out one excellent episode after another. Many of them masterfully interweaved.
The 4th season kept the pattern going. And the Highlander team continued with exploring characters, and changes. While the concept was limited (who will Duncan meet from his past this week?), as such, we were introduced to the "dark side" of Duncan Macleod. If because of what the cast and crew had working together - the show continued on putting out either excellent, or good shows. While this turn was interesting, the somewhat dark side didn't always click with fans who saw Duncan as a role model and hero.
The 5th season found the writers trying to vary the previous season's problems. With many espisodes Duncan changing (often not using the sword at all), and sometimes mystical elements involved (which turned some fans off). But the professionalism continued.
By the 6th season, the show was hit and miss, and it seemed at times like it was soon to reach the end (which it did). The final episide was somewhat sentimental, but at the same time, fittingly obscure.
Highlander - Raven continued with the charming character of Amanda. But without Paul, the series didn't have the same backbone, and ultimately waned.
Adrian Paul would later reprise the Duncan MacLeod role in the film Highlander: Endgame, which is the best sequel as the films go (and on par with many TV episodes), but alas, the film did not fair well at the box office, as the previous sequels (2, and 3) were so bad, the mainstream stayed away.