I can't decide so I will list three that are of different eras.
1973 Viking 440 Vigilante SS, the first sled that I put serious miles oon. It had a 440 Kohler leaf blower, a wide ski stance, factory studs in the track, and a para rail suspension. I rode it about 2500 miles a year for a couple of years. It was my sled when I worked as a guide in 72, 73, and 74.
1980 Polaris Centurion, my first liquid cooled sled and also my first triple. 32 inch ski stance with leaf springs, a rubber track with 24 studs right down the center, and about 5 or 6 inches of travel in the rear. It was the first sled that I took on backpack trips and it was fast, the first sled that I had over 100 mph (on the speedo anyway ). It's fun factor was great.
2012 Polaris 800 Switchback Assault, I can ride it on rivers and lakes, across fields and forest roads, and it is also very good on groomed trails, I have had many 300 mile days and feel great after a long day of riding. I am sure that I will be able to add to this list in the coming years.