I've got 2 - Explorer 213 because I view the whole area as 'my playground' and Explorer 442 because you don't often find that many contour lines so near together
Well I currently still have an academic subscription to edina digimap, so can make my own custom OS maps for wherever (if I have the time and inclination)
But otherwise, it's got to be your local area hasn't it :D ..not that I can remember what number that is
Because it's where my in-laws live and I love spending time staying there but away from them! Seriously, it was my father in law who first got me out on a mountain bike so i'll always have a soft spot for those eye-opening first rides that he took me out on. More recently, i've gotten into walking and I love the harsh scenery of the Dark Peak. Like OL24 too as it contains the Goyt Valley which is another nice area up there.
Funilly enough, i'm not that interested in the London South OS map of where I live!
The OS map is pretty useless anyway in the south as a lot of the trails aren't on the map and are cheeky local knowledge ones.