My pick of 10 would be:
Roof
Windshield
Soft upper doors
Rear panel
Winch
Arched control arms
Mirrors/side and rear view
Stereo of some sort for tunes
Bucket of recovery items/tools
Snow plow
The first 6 should come on the machines for what they charge.
This is a list of in my opinion requirements to always ensure that you have a good fun safe time. I would recommend helmets as well but some say seat belts and roll cages are enough.
The bucket of recovery tools I think deserves its own single item slot. What people choose to carry is different for everyone. It just depends on where you ride and how you ride. For some people they need just a few straps or maybe a shovel and they are set. Others need spare parts, 3 days worth of food, some flares, and a port a potty. It just really depends on where and how you drive.
As for me, I have the OE roof and I like it. I have the SuperATV flip up windshield and if I could do it over again I would have invested in a flip up glass windshield instead. I have the OE soft upper doors. I think I Iike them but I haven't even installed them yet. I have a OE soft back panel and I like it. It cuts down on dust. I have a champion 3500lb winch with synthetic rope. I purchased it when I was in a bind, I needed it immediately so I could install it for a trip the wife and I were taking. I used it once for recovery and it worked. I used it a few times for snow plowing and it worked. I do not feel as comfortable as I would if I had the opportunity for a better weight rating and better brand but for now it works. I love my highlifter arched arms. I went cheap on my mirrors and so far I don't regret them. I just really appreciate the ability to quickly and easily see my surroundings. My tune bar is 2 Aomais Go speakers. I really like them alot. I do plan on a full sound system at some point when money is available. My recovery items include straps, shovel, farm jack, flat repair plugs, 12v air compressor, and a craftsman tool set with odds and ends like tape, wire cutters/connectors. No spare parts yet
lastly is my snow plow. 72" Denali from the club store. I have never used anything else but I can say I love it.
Now my personal list extends way past this. But if I have to pick just ten then this is my list.