The 8 Plus... but I'm biased after a year with the 6 Plus and 2 years with the 6s Plus. I'm a minority user with big hands who can use the Plus one handed... size is just right.
Too bad Apple didn't release an X Plus... I may have bitten instead of ordering the 8 Plus.
To me it’s the X. I love the 16:9 ratio’s in the 8 and 8 Plus, but I don’t use my phone in landscape very often so I’ll appreciate the extra vertical real estate and smaller form factor for my hand that the X provides.