I'm waiting until next year to upgrade for a variety of reasons.
1. I already upgraded 3 phones this year, two iPhone 7's for my wife and her sister and a Samsung galaxy s8+ for my son. My Verizon bill is now way more than I'd like it.
2. I'm sticking with iOS 10.1.1 for now for the wallpapers I use. They don't work well past 10.1.1.
3. I'm not that impressed with iOS 11. While it has some nice new features, there's much I don't like, including the new app store. On my Wife's phone, I like to never found the purchased list. The app store seems to be too busy with too much junk I don't care about.
4. The iPhone X screen size isn't what I would want. It's too narrow and tall. I'm hoping next year brings a larger version that's wider. I do feel it's a bit overpriced, but it is what it is. Depending on next year's price of a larger version, I might opt to upgrade to this year's iPhone 8 Plus.
5. IOS 11 kills all 32 bit apps. I still have several I like and use, and can't yet find suitable replacements in the App Store.
If I were upgrading this year, it would be the 8 Plus. The X just doesn't thrill me that much to pay $150 to $200 more for. I don't need wireless charging that I must pay extra for gadgets in order to use it. Plugging in to charge is fine with me. I would like the animojis and better camera, but not enough to pay that much more for.
Update: Since the OP wants just one in his list, it would have to be
#4 , The upgrade is not compelling enough.