In my opinion, Portal IS the best singleplayer part of the package, if you assume that most have played Half Life 2 already. I loved Portal, and it had a bigger effect on me than the Episodes. That said, I really did enjoy Episode 2, but it was too short, and they explained bugger all. Either they need more than just a third episode, or a full game sized episode 3 to satisfactorily explain it.
I agree with most of that except for needing more explanation. Have you only played Episode 2 once? You may find more things in it after looking closer.
One of those things is the fact that Aperture Science is involved in the Borealis. Some speculate that means the significant power there that needs to be destroyed is the portal gun from Portal.
If that's true Episode 3 could be filled with great puzzles using the portal gun. I'm thinking much if not most of Episode 3 will take place inside Aperture Science labs and the Borealis. I certainly hope will get to use that chopper a lot on the way there though.