The definitive DR WHO serial for me will always be 1988's 'Remembrance of the Daleks'.

Loved all the allusions to the Doctor's dark history and the 'Hand Of Omega'...

the sinister British neo-nazis collaborating with the Daleks subplot...

all the references back to the shows past...

Ace kickin' some Dalek ass...

existential angst over tea with Jeffrey from the Fresh Prince...

and of course when this cliffhanger happened I nearly crapped myself*...

A quarter century ago, really...? bloody hell I feel old now ;-)

(* They need to bring back the multi-part serials. It's not the same without those cliffhangers IMO)