TNG. I would say that the original series needs remastering except that they're already doing that. The problem as I see it with TNG is that for someone like me who was introduced to TNG starting with the 4th or 5th season, the contrast between the quality of the effects in the 5th season and that in the 1st season are somewhat jarring.
It wouldn't have been that bad if the show had had a unified look in terms of effects but the dramatic improvements of the later seasons should be applied to the earlier seasons. The later seasons are good, although they could use a few tweaks here and there.
I think it would be cool if they scanned the model of the enterprise-d and created a digital ship for the shots. That way they could show us some views of the enterprise that we never saw when it originally aired on tv.