Fisher's HMS Incomparable.
Only a concept, not a full design like others mentioned here, but 20" guns and 35 knots ... you want to see it, don't you.
(I would never argue this was 'the best warship' concept, but I note use of the descriptor 'cool' in the OP).
Returning to a more sane world, I'd nominate 'X4' of 1905.
A 'fusion battleship' (i.e. fast battleship) with 10-12" (or 8-13.5"), 25 knots, armour as Bellerophon but with better torpedo protection.
Only a concept, not a full design like others mentioned here, but 20" guns and 35 knots ... you want to see it, don't you.
(I would never argue this was 'the best warship' concept, but I note use of the descriptor 'cool' in the OP).
Returning to a more sane world, I'd nominate 'X4' of 1905.
A 'fusion battleship' (i.e. fast battleship) with 10-12" (or 8-13.5"), 25 knots, armour as Bellerophon but with better torpedo protection.