Opinions ahead
The Christmas Invasion
Tricky one because it feels like a natural extension of Series 1, especially with the return of Jackie, Harriet Jones, etc. But typically I view it as the start of Series 2 because it’s Tennant’s debut.
The Runaway Bride
There’s a direct continuation of the Doctor’s grief over Rose, so for me this is the conclusion of Series 2.
Voyage of the Damned
Aside from Tennant’s debut, I find it strange to view a Christmas special as the start of a series… so again this comes at the end of Series 3.
The Next Doctor
Technically this is part of the ‘specials’, I guess? But this one feels a bit like a lone ranger. I view the specials largely as ‘the 2009 specials’ (even though The End of Time spills over), so The Next Doctor is just sort of standing on its own. I like that Series 4 ends with the Doctor alone in the TARDIS.
Planet of the Dead
The Waters of Mars
The End of Time
I don’t really view these as part of Series 4. As I said before, I view them as ‘the 2009 specials’, or just ‘the specials’.
A Christmas Carol
The end of Series 5.
The Doctor, the Widow, and the Wardrobe
The end of Series 6.
The Snowmen
It’s slap bang in the middle of Series 7.
The Day of the Doctor
The Time of The Doctor
Like the 2009 specials, I view these as their own distinct thing.
Last Christmas
End of Series 8.
The Husbands of River Song
The Return of Doctor Mysterio
No real opinion, although Husbands without Mysterio feels like the end of Series 9 for some reason. But I don’t care either way.
Twice Upon a Time
Its own distinct thing, or as a coda to Series 10.
Resolution
Whilst this feels more fitting as a Series 11 finale, I view this as its own distinct special.
Revolution of the Daleks
Eve of the Daleks
Legend of the Sea Devils
The Power of the Doctor
Not part of any series. They’re all just distinct specials, the way the 2009 ones were.
The Star Beast
Wild Blue Yonder
The Giggle
Just 3 specials for the 60th anniversary. Self contained mini series. Not attached to a series.
The Church on Ruby Road
Feels like the start of Season 1 for obvious reasons.
Joy to the World
No idea. Probably the end of Season 1.