What about Boxers?
They are known for being "Puppies for life".

My girl is only going to be 2 this July (17mos. right now), but she's a nut!
We're starting another obedience class right now and then we're going to move into agility.
They're a medium breed so they can do agility quite successfully and they are QUICK!
Theres a woman that i spoke with one time named Renee, her boxer has achieved all kinds of agility titles so far and really doing well!