My vote for the best quickenings would be the daul quickening when Duncan took Kronos' head and Methos took Silas'.
The second best quickening would have to be Kalas' quickening in Finale part II.
I have to admit that these are my faves, but I also like Connor's first Quickening from HL2, The Kurgan's alley Quickening, and Connor's car park Quickening.