[Matrixbeast]

Alien Force was fun, but it wasn't until the later episodes that I actually enjoyed it. Ben having perfect control of the Omnitrix (Or his 10 aliens, that is) killed a lot of tension, and it removed the need to ingenuity.
Original Ben wants Four Arms and gets Ripjaws? He has to figure out a different way to win than just "Rush him and overpower him."
AF Ben 10 wants Humongosaur? Gets Humongosaur.

I know the glitch thing could (And sometimes did) get old, but it made Ben stay on his toes, because he didn't always get what he wants.

However, the story end of the spectrum was better and it did get more interesting as the show went on, and I do enjoy Alien Force (Except for that whole "Magic isn't real, you're just an alien." thing they did to Gwen then retconned out in Ultimate Alien).
But the fights were just dull in Alien Force, since Ben fights in a very linear and straightforward manner. Jetray, get in close, Humongosaur (Ben used Brainstorm so rarely it hurt. I think he's only beaten by Ripjaws for least used of the default 10 aliens).

As for Gen Rex, I could never get into it, personally.