That's an interesting question, mainly because there isn't much of significance in the family coaster bracket in the UK. Dragon's Fury at Chessington probably takes it for me, with honourable mentions going to Vampire at Chessington, and Flight of the Pterosaur at Paultons.
It's a shame actually, as I'd much rather ride a good, fun, family oriented coaster where everything comes together well, rather than the poor excuses for "thrill" coasters that we've been getting in the UK lately. Unfortunately, most UK parks seem to be about as good at building good family attractions as they are at building thrill ones
. Thumbs up to Paultons for the best new family coasters I've seen over here in a long while.
I do wonder whether some of these polls would benefit from an "other" option, as inevitably someone will have something obscure and unexpected as a favourite.