I loved Merlin as a series, and all of the main characters! It built up in intensity over the seasons, and the characters all matured in plausible ways. Plus the show makes good use of the found family trope and the "kind but badass hero" trope, with lots of humor sprinkled in. It's a bit lacking in the female main characters and diversity departments, but the women they do have on the show are so great. S5 takes place after a jump in canon time, and is a bit different from the rest of the series - I don't want to spoil anything, but it's definitely more anxiety inducing and darker which I thought worked well. The fandom definitely has problems with how it all ended. It was sort of a cliffhanger.

Hope you get to enjoy :) I have been really impressed with the way Colin Morgan tackles completely different characters.