The 6en/40xp missions produce the fastest xp gain in ay given time cycle, with the exception of the MW instants played in the first hour with a minimum of a 24/25 bar.
Although, a better xp gain to play the first 25 points in the 12 and 8 hour long bakes, and play the 6en/40xp missions on a 12 hour cycle, once every hour. In any case, you have 2 hours and 5 minutes before your meter coasts at 25 eng, from a zero state. So you don't have play every hour, and the beauty is that you can play them in tandem. Which gives more flexibility, and it only comes down to not letting the meter coast at 25.