The S10, S10+ are both very good phones. I don't know why the sales numbers [allegedly] are that much better than last year. The XS Max is also a good phone too.
Right now, it's all about preference, although in the "real world" experience, the S10, S10+ perform slightly faster than any iPhone. It will be interesting to see if that will remain considering the Android OS tends to slow down the user experience over time even if you don't update it.
Personally, I was on the fence whether to choose the XS Max or S10+. I went with the XS as there are more accessories available for it and is better built to withstand falls (as many tests prove). Still, both phones are good.
I might consider the Pixel 4 when it comes out due to its low light functionality. Cost wise, the One Plus is also a contender mainly due to reduced proprietary software like Samsung has. It would be ideal if Samsung made a "vanilla" smartphone.