Prime

I'd say play ME3 w/o the EC first and then install it and go back to right before Cronos*. Have only gone through the Refusal ending so far, though. Have 3 more variations to go... Have to admit that the "slideshow" concept mentioned in leXX's post sounds like a cop-out (edit: turned out to be less obnoxious in execution that it sounded).

*on subsequent playthrough, noticed you won't have to go back that far afterall. Just play through from last save on earth before hitting Citadel (don't know if you're doing it console/PC). Of course you can just install now and watch the "old" endings on YT.