Other stouts to use in a 'Guinness cake' I've previously had success making a guinness cake (http://www.nigella.com/recipes/view/chocolate-guinness-cake-3086), and, whilst it was tasty, I figure there has to be a better stout to flavour it with. Any suggestions?
Guinness is in a style called "Dry Irish Stout", so anything on this list would probably substitute well.

It sort of depends on what you felt was missing from the cake.  We could work on narrowing it down to stouts that were stronger in those flavors.

Oatmeal or milk stouts as styles would be sweeter than Guinness, some stouts might play up chocolate or coffee or roast/burnt.  Porter is a really similar style that takes well to interesting spice combinations like vanilla and anise.  Bourbon Barrel aged stouts will have a wood/vanilla/whiskey thing going on.  You could make some interesting cakes.