TV’s Best Season (not series) Finales (off the top of my head and in no particular order)

The West Wing Season 2 (Mrs. Landingham’s funeral, “will you be seeking re-election”)
The West Wing Season 3 (the war of the roses, Mark Harmon dies, the assassination of Sharif)
Lost Season 3 (charlie’s sacrifice, flash forward)
FRIENDS Season 4 (Ross says the wrong name at the wedding)
FRIENDS Season 6 (Monica and Chandler’s engagement)
How I Met Your Mother Season 1 (Ted makes it rain)
How I Met Your Mother Season 2 (the wedding)
The Office Season 2 (casino night, Jim and Pam’s first kiss)
Gilmore Girls Season 1 (Max’s proposal)
Gilmore Girls Season 3 (Rory’s graduation)
Buffy Season 2 (killing angel)
Buffy Season 3 (graduation)
Buffy Season 5 (Buffy dies)
Buffy Season 6 (evil willow and the yellow crayon)
Veronica Mars Season 1 (solving the murder, who’s at the door?)
Slings & Arrows Season 2 (Macbeth and Romeo and Juliet open)
Brothers & Sisters Season 1 ( the engagement party, goodbye to Justin, jumping in the pool)
Grey’s Anatomy Season 1 (Addison’s arrival)
Grey’s Anatomy Season 2 (Denny’s death)
Grey’s Anatomy Season 3 (Christina’s wedding)
Prison Break Season 1 (breaking out and starting to run)