This is an important question..
Bitter - 500ml - has 11.5g of carbs and it seems to make no difference if it's canned, draught or keg.
Bottled lager - 500ml - is better at 7.5g
Brown Ale is worse at 15g, but Mild is lower at 8g and Pale Ale is somewhere in between at 10g.
Stout (bottled) is terrible at 21g and my book lists "strong ale" at 30.5g - so Old Peculiar will make you all peculiar.
If you want to really send your readings skyward, however, then Vintage Cider beats them all at 36.5g
(all figures from the Collins Gem Cab Counter - not personal experience).