There are some fairly mainstream non-pale bitters that I like a few of which have been mentioned :-
Adnam's Bitter
Fuller's ESB
Everard's Old Original
Holden's Special
Banks's Bitter
Robinson's Best (now called Unicorn) and Double Hop
Hook Norton Old Hooky
Woodforde's Wherry, Nelson's Revenge and Admiral's Revenge
Triple fff Moondance
Harvey's Best
Brakspear's Bitter
And a recent one Redemption Urban Dusk which is a dark bitter.
My boring list would include
Fuller's London Pride
Well's Bombardier
Young's Special
Courage Best
Courage Directors
Tetley Bitter
Sam Smith's OBB
McMullen's AK
McMullen's Country
Draught Bass
Greene King Ruddles
Plus many others no longer available!