The first season is pretty cookie cutter. Duncan meets bad guy, beheads him, next episode.
After that the show fleshes out in its own direction well
Season one the show really hadn't discovered itself yet but still had a great early 90's straight to VHS charm about it that I really love. The episode where he fights the mountain men(the Beastmaster actor) has my favorite fight scene in the whole series there in the end with the axe. That was so savage. It's a cookie cutter formula but it's one that works for me, the show did a good job of the simple task of building up the bad guy and building up the final showdown. There were a few fights in there were Duncan was in serious trouble of losing, probably closer than he ever came in the seasons after. Grayson was also a great 1 time character. Even the bad episodes of season 1 I like plus I noticed a lot of little things foreshadowing everything that happened with Richie later in the next season, it was a much better written show than what I remembered loving as a kid.
Season 2 starts off amazing, peak of the show with some of my favorite episodes. Then it really drags with a bad stretch of some of the worst episodes of the series before picking up and finishing ok.
Season 3 is where the show really hit it's stride and became like one of the coolest things going at that time in the 90's. The episode where Duncan unleashes the dark quickening from his Shaman friend he has to fight, then turns evil and wreaks a bunch of havoc like a bad ass villain before killing one of his best friends, then has to fight himself after recovering his family's sword(from the first episode of the season where he goes back to Scotland to find it), the spirit of his friend that he killed helps him defeat his own evil self in a mirror image showdown. That is really fucking bad ass shit right there brother. Some of my favorite 90's TV ever. Also just don't know if it gets any cooler than the origin story episode for his dragon head handle katana from that same season where it shows him in feudal Japan how he got it.
Season 4 started off good too and has the peak episodes the Four Horsemen two parter, easily better than any of the movies including the 1st one. Methos becomes one of the best things about the whole deal, a 3,000 year old immortal from the bronze age, just awesome.
The Four Horsemen season 4 episodes were like the mid-season finale and after that the show was really dead, the rest of those episodes in season 4 was like jump the shark stuff. I think that season was 1997 and the show's decline matches an observation I've always had about how cool things from the 90's really kind of died out in the transition from 1996 to 1997. With Highlander the series being from 1992 seemed like one of the coolest things ever in 1996, but by 1997 seemed like a dated relic from a previous era, that was how fast that decade moved.
Season 5 pretend it doesn't exist, shhhhhh.
Throughout the show you get some pretty spectacular sword fight scene choreography and the opening intros never get old. Also love all the drop in special guest 90's actors like the one bad ass from Last Of Mohicans, Beastmaster, Randall "Tex" Cobb, several others I'm just not remembering I watched through the whole series last September and really really loved it.