I'd say you can go wrong with a pumpkin ale <shudder>, but since 180 likes the idea, 'tis a moot point. I will put in a vote for brewing things that appeal to you, and not, say, making IPA's if you're not a hop head (I'm not, and so I don't).
Dark beer, or specifically Guinness? Guinness is (IMHO, at least as exported to the states - I hear it may be different at home) a fairly thin and bitter member of the dark beers, many of which are more robust and less bitter. I love Samuel Smith's Oatmeal stout, personally.
Pretty much any porter/stout sort of thing should be good - winter is coming on. By March or so, think ahead to brews for drinking in summertime.