Honestly, the TV series is better than the movie. The movie is Epic, but cheesy. The Series fleshes the whole thing out fairly well, except for the last season, which was kind of phoned in and stupid. It has it's highs and lows, but was overall really good.
The Source, unfortunately, is an attempt to do the post-apocalyptic quest epic, which they'd been trying to do in some form or another for a while now but always failed at.
Obviously, Highlander 2 was their first attempt at post-apoc. Sean Connery is almost worth all the stupid in that one.
Originally, they had intended Season 6 of the Series to be post-apoc, but shied away from that for budget reasons and because Adrian Paul wanted to leave the series. The while Arhiman plot at the end of season 6 was supposed to end the world.
The cartoon series actually did post-apoc pretty well. It had an interesting, evocative world. Unfortunately, it suffered from the same fate as most cartoon series, no real planned arc and cancellation.
The Anime did it best.
You can see parallels between Ahriman and the Guardian, particularly in how Duncan had to refuse to fight in order to win. I think that the Source is basically what they wanted to do with Season 6, but worse.
Really, they need to learn not to tack post-apoc hellscapes onto live-action Highlander movies.
3 and 4 were actually passable, if mediocre. But both stuck to immortals in modern times, and the basic good-vs-evil epic structure and so they worked.