Democrats: Senator Clinton and Barack Obama
Obama would make the ticket more appealing/personable and the call for change would be met. Also, the experience would season Obama for a stronger run in 8 years. The ticket would attract the most Democratic voters.
Republicans: Rudy Giuliani and Fred Thompson
Thompson would please conservatives who are not quite sure of Giuliani but would like him to see him in the WH and his demeanor would make the ticket formidable. Giuliani knows Thompson would have no interest in competing for the Presidency years later.
The tickets would maximize the Parties' strengths and would position them well for 3+ months of incredibly fierce campaigning.