I use a lot of flagships and Saumsung UX is my least preferred which stutters the most and lags here and there. It's improved a lot in S8 compared to S7 so that's good sign.
Based on my usage of phones I owned, here is what I prefer in terms of buttery smooth performance. This just takes UX perf and not features, other advantages phones have in other areas.
1. iPhone 7 Plus/Pixel XL
2. Xperia XZ premium
3. One Plus 5
4. U11
5. S8+
6. G6 (my least preferred)
I haven't used flagship Moto phones but have G5 plus and based on how well that performs that will sit near OP5, U11 etc.
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