First time, 4 is probably hardest or second hardest bc of masochism. For players that have more developed rosters and are more experienced, it's pretty easy. 1 and 2 are actually fun bc of power shield. 3 is mildly annoying. 5 and 6 are similar to 4 bc it will be much easier for a more experienced player, but quite hard for a newish player. 6 also has the collector which is something by itself. 645321
It'snog about hardest and easiest.It's about storyline, if the do what you are saying storyline will be messed up.