Every host appreciates a guest who brings a gift of alcohol to a party (at least, if that party is serving alcohol). The type of alcohol you bring will depend on the type of party, your relationship to the host, and whether you intend the alcohol as a host's gift or for immediate consumption. If you intend the alcohol you bring to be served at the party, ask the host beforehand what kind of alcohol best fits his needs.
Clear alcohol, such as gin or vodka, can be a good choice for parties, as it is extremely versatile. You can purchase clear alcohol in a variety of price ranges, from bargain to extremely high-quality. It's a good idea to bring a variety of mixers if you choose to buy clear alcohol, especially if you intend for it to be consumed at the party. Tonic works well to mix with gin, while juices work better for vodka. Anything from cranberry to orange juice works well for the latter, which is one of the most versatile alcohols.
Dark alcohols, for example whiskey, work better for smaller affairs such as dinner parties. This kind of alcohol is better savoured as an after-dinner drink, and the higher the quality, the better. Ask your local off-licence to recommend a high-quality scotch, like Glenfidditch, for a particularly special gift. Other dark alcohols, like rum (which can be purchased in a light form as well), can work for larger and rowdier parties; ask your hostess before you arrive what she would like you to bring.
Wine and Beer
Perennial favourites, wine and beer work well in almost any context. Wine is better for dinner parties, housewarming parties, and in any case where you want to present a more personalised gift. A bottle in the £6 to £13 price range is usually appropriate, though you can, of course, spend up to several hundred (or even thousand) dollars for an extremely special bottle. Beer, in general, is less elegant but more festive, and can be a welcome addition at a larger party, particularly towards the end of the evening when supplies may be running out. Microbrews or foreign beers can be a good choice for host's gifts, especially if you know that your host's tastes are more in line with beer than with wine.