I am looking to buy a new sled and I am wondering which pro-x model is the most reliable

The 440 fan is most reliable, but any of the liquids will do great. Pretty much all newer Polaris motors are reliable.

Don't buy a fan cooled

600 liquids are the way to go. My oldest daughter had a 440 fan and it WAS NOT a good trail sled. She now has a 550 fan and that is a much better trail sled. Of course my 600 Pro-X2 is the BEST trail sled.

Tom

The 600 is the most reliable no question.

the 600 first then the 550 then the 7 then 8 then the 440.

i have a 03 prox 600 and a 03 prox 800 love them both but the 600 is the perfect trail sled good holeshots and midrange will stay with the 800 up to 85 to 90. only problem i have ever had with both sleds is they both busted the bolt on the lower gear in the chaincase anyone else have this problem? because it happened on both of mine. so i would replace that before you get stranded in canada like i did.

My 1999 700xcsp broke the lower sprocket bolt at about 5000 miles. It blew through the chaincase leaving a big hole ....My 03 pro-x 600 has been a great sled(4700miles),one of the best chassis and motors since the 1989 indy 500. I thought they solved that lower bolt problem? The only problems the 03 600pro-x has had is one blown belt and a broken drive clutch spring. I also replaced the inner idler wheels when I had shocks serviced at about 2500miles.