The short answer: No UTV is “the best one” for all terrain types and rider preferences. Each has strengths and weaknesses, and it boils down to personal preference in most cases.
Deciding which UTV should be your first is a very tricky decision, and most people get it wrong. A common mistake is to buy an older, used model, or a knockoff brand that is less capable than the market leaders, to keep the initial cost down. Those people almost always end up wanting more within a year, and it ends up costing more than if they had bought what the really want to begin with.
In WA you have single track, ATV, 4×4 trails, and service roads. There are no trails designed for UTVs and probably won’t be anytime soon.
Even a 50″ UTV will have extreme difficulty on most ATV trails, and simply won’t fit on others (48″ bridges at Tahuya, for example). A 50″ UTV will be tippy and uncomfortable on most 4×4 trails in this state. Also they are not allowed on DNR trails that are designated for ATVs (50″ or less).
If you’re going to take it out of state a lot, you might still consider a 50″ trail model because there is a lot of stuff in Idaho and Utah where such a machine will do very well and anything wider will be very limited.
But if you plan to stay mostly in WA, then it really doesn’t matter much which UTV you get in terms of which trails you’ll fit on. Yes, an S model is more nimble with its shorter wheel base. But people go on those same trails with 4-seaters. It’s just harder, and they go slower.
My suggestion: Think about the kind of riding you want to do (dunes, mountain forest, rock crawling, open desert, etc), and think about what features you want your UTV to have, and find the machine that most closely fits that profile and you feel is badass. Then go sit in one.
See if you can find someone who will take you as passenger in a few different UTVs in a few different types of terrain. THEN spend your $20K or whatever.