Would say either who leads the Primary polling or whoever is actually in the race or likely to be in. For example in VA, Allen generally has the better numbers against Warner (though Warner is still well ahead) however I wouldn't put the poll with Allen in there because he isn't running yet, nor i their any indication he will run.
OK - but it's important to point out that there are a lot of states that haven't had primary polls.
For the most part you won't see all that many General Election polls in states their are no Primary polls. Granted we have seen general election polls in VA, and no Primary polls, such as Allen going up against Warner, and we haven't seen a poll with Allen in a Primary, but Allen isn't even in the race. Other times you might not see a primary poll, but it is obvious who the frontrunner is so go with that (Udall in NM for example)
As a whole I would say this:
If the General Election Poll has more than one candidate from either or both sides go with the candidate who is declared or likely to jump in if the others are not (it makes no sense to include polls of candidates who likely won't run
If the General Election Poll has more than one candidate from either or both sides, if Primary polling does exist and the candidates in those polls are either declared or likely to enter go with the candidates who are leading the primary polling
If the General Election poll has more than one candidate from one or both sides, if more than one candidate from either or both sides has declared or is likely to jump in and their is no Primary Polls, go with the frontrunner (for example again with NM, you don't need Primary polling to tell you that Udall & Wilson are their Parties frontrunners)
