Out of all airlines, United has given me the most problems. Due to my "priority status" with most airlines, I am likely sheltered from the brunt of the harassment, but I often witness other passengers dealing with toxic attendants, overbooking situations, crew movement taking priority, and so on.
Emirates is my primary international carrier. However, for many of my international trips out of DFW, United is one of the only premium carriers available.
Truth be told, my best experiences have been with Virgin and Southwest.
Virgin is now my go-to carrier when flying longer-haul domestic between destinations within their market. The Wright Amendment previously crippled Southwest's usefulness but, since it's removal, I find myself flying with them more and more often. Love Field, due to its size and location, provides such a painless travel experience in and out of Dallas. The alternative is flying via DFW, which necessarily adds over an hour of traffic delay each direction and maybe half an hour of TSA delay, even with pre-approval, when compared to Love Field. Saving 2.5 hours each time I fly is quite the value proposition.
Given these United revelations, for international flights, I will try to book with other carriers more often - Luftahansa, Japan Airlines, Korean Air, Qatar Airways, AeroMexico, etc.
TL;DR: United truly is the worst. Virgin and Southwest are good. Use Emirates for international.