The last two seasons continued to excite because our characters, Ben, Gwen, and Max had shown that they had made use of what we had seen happen to them (ex. Gwen's magic, Max's Plumber Gear, Ben's new aliens).
Probably my favorite part was that every voice actor for the aliens were so incredibly perfect. Diamondhead, Heatblast, Grey Matter, Cannonbolt, XLR8, Ripjaws, and Wildmutt (Dee Baker made noticeably different Wildmutt sounds in OS than UA or OV) were the most memorable voices for this series that I'm afraid we won't hear again, (I didn't catch Destroy All Aliens)
I sort of treat the first two seasons of AF differently from the third, which I group with UA in my mind. The seriousness was perfect for this next stage of Ben's life. Although it was surrounded with questions like how/why did Ben take off the watch, what happened to Kevin, and can Ben control when he turns back to human/ does the omnitrix recharge? *phew
But the aliens were likable and Dee Baker had them all under control. Character development was important as always, and with the Maestro leading the show, everything we saw paid off in the finally (except Alien X)