I have many and it's really hard to choose one, but if I really had to, I'd stick with It´s the End Of the World (As We Know It)...it was just so amazing, shocking and great written...the whole season 2 was great, I think the best. And the second favourite episode for me would be this season finale - Losing My Religion.
But really, I enjoyed many other episodes - Freedom, Time Has Come Today, What I Am, I Am a Tree, The First Cut Is the Deepest, Now or Never, Elevator Love Letter and more...

I don't know about favorite episode, but my favorite season is 2 - It had all my fave characters - George, Izzie, Burke (preston) and Addison. But an episode I found extremely intriguing was the episode George and Izzie have their affair.

My Favourite episode is Stairway to Heaven (Season 5, Ep 13). The ending of it is amazing!! i cry every single time i watch it. The song is great too. (Drifting Further Away by Powderfinger).
I have many other favourites though- Death and All his friends, Shiny Happy People, A Change is Gonna Come, There's no I in Team.
My favourite season though, is definitely season 3.

i dont have 1 i have 2 so here thay are
the frist one is the bomb sean i love this episode idk y i just do

and then this 1
the episode is were meredith drowns and derrick goes in after her love that part

And my new favourite episodes - definitely Sanctuary & Death and all his friends!

Season 6 episode 8 "Invest in Love" So sad and sweet!

definitely the season two finale. it was epic with denny dying, meredith and derek hooking up at prom, and izzie quitting....wow...and dont even get me started on the music...chasing cars <3

but i must say that the season 6 finale was pretty amazing too <3