I voted for Bad Wolf/Parting of the Ways. However The Caves of Androzani is second in my heart.
There is so much atmosphere in Bad Wolf/Parting of the Ways.
The last adventure of 9, Jack and Rose, Jack
The 9th Doctor being his cavalier self. Doing all he can to defeat the Daleks, not knowing this was his final battle.
Jack being heroic in fighting the Daleks. The Jack of Doctor Who i prefer. Compared to the one we later see in Torchwood.
Rose becoming the Bad Wolf and coming to the Doctor's rescue. This of course with the assistance of Mickey and Jackie.
Lynda with a Y. The companion that never was. Oh how my heart ached the first time seeing her die T_T. It still bleeds when I rewatch the episode.
Many other things I enjoy about this two parter. I really wish there was more 9th Doctor material tbh. Really sad Chris Eccleston decided not to reprise his role for the 50th anniversary episode.