Solar Thermal Propulsion beats all for /sfg/
A solar thermal rocket using a concentrator made from the materials being developed for solar sails and using hydrogen as the propellant, will give higher delta V at lower cost than - Chemical rockets, nuclear thermal rockets, solar electric propulsion, or nuclear electric propulsion.
With Isp 900s and mass ratios of just 4, you get delta v of over 12km/s, with aerobraking 50-60 days to Mars.