I'd have to say the Heritage 1, Heritage 2, Heritage 2 Legend, and Legend 2.
Mainly because these Kirby's offer the very best in every conceivible attachment ever devised by Kirby.
Starting with the Heritage 1 you could get the Hair Clipper, the Handy Butler, Rug Renovator, Miracle Head,Inflator, Waxer, ect. which would all work with any of these Kirby's as their Safety Switch was/is the same for all.
Not to mention the avalibility of your choice of the regular Sani Emptor, or Mini Emptor, with the redesigned papaer bag system.
Plus, these Kirby's used 2 speed motor, and the 4 row brush roll, that not only grooms the carpet and raises the nap better than a 2 row,,,it also helps to pull the Kirby along, making it easier to push.
Not only that but Kirby even made avalible a smaller 500 series Rug Nozzle, for more concentrated cleaning, or for anyone that prefered the smaller head Kirby.
Plus, you had the choice of the 3 different rug nozzles,,the Classic series large bell shaped nozzle, the Heritage 2, airplane wing shaped nozzle head, or the 500 series small head.
What more could a person ask?
The handle of the Heritage/Legend machines is even designed to be user friendly.
Altho i will always love the Classic series, and the Tradition for their regal look and color (especially when new, or Factory Rebuilt),,the 2 row brush roll does make these machines very hard to use,,especially on thick heavy carpet. Imho of course.
Ask any older person that bought or owned a Classic series Kirby back in the day,,,the first thing they will say is, IT WAS SO HARD TO PUSH!
If you own a Classic or 500 series that is hard to push, put a 4 row brush roll in the nozzle,,,you will like it much better.
Imho, of course. To each his own!