LOL! FF versus, original Defenders. Give me a break. This is almost as bad as Wolvie versus The Living Tribunal. Let me break it down.
Strange takes out invisible woman. She is the strongest member (she's taken on the rest of the FF before and nearly killed everyone) and her support role makes the FF Stand up against much more than they normally could. Namor is smart, if brash at times, But he'd take out the King, and ruin any back up Reed could muster. Hulk versus Thing, sure Thing's had his moments, but Strange'd be done so fast he'd be able to help with this, if not already providing support for the rest of the team (remember the FF usually rely on strange for support versus magic.) After this it turns into a mop em up. Not even worth 3 pages in a comic book.
You really think a stretchy guy (no matter how smart he is,) a fire guy (who's never been able to hurt any of the Defenders.) A tough guy (who isn't as tough as anyone on the Defenders.) and an invisible woman could take the Defenders!
Which by the way is comprised of the Sorcerer Supreme, Earth's first and mightiest mutant, Weilder of the Power Cosmic, and The world's most endless supply of raw power?