Gemma, by far. She made week 1/2 memorable and basically provided the only source of real conflict.
People calling David an undeserved eviction is silly. Any eviction at this point is not "undeserved", because its basically the end. This is not like it would have been if Tim left a week before the end. There was nothing overwhelmingly distinctive about Dave that makes his eviction undeserving.
i think gemma was the only one to get truly boned by bb with that whole 'housemates choice' business, but so far lisa is the one that seems like the greatest loss. maybe katie too.