Heat Goes to Cold by
turned_earth/Amandr is one of my favorite all time HL stories. Methos mourns Alexa.
Archangel Redeemed by V.Watts (
maygra ??) and
macgeorge1 is one of the first HL stories I read. It's pretty self-explanatory - the authors' rewriting of Archangel.
Abomination,
Witness and
Broken Chains by V.Watts - a Methos-centric series. The latter two stories are het.
Winterborne (and
two sequels) by Beck McLaughlin (
beccaabbott ??) another favorite. Future!Dystopia!Fic featuring Methos and Duncan and a variety of great original characters.
Episodic Sadie series by BBC is another great comedy, featuring an OC with a... special... relationship to Methos and Duncan.