Say what you want, Adonis Creed did not deserve to win. As a character, as an individual, as anything. I mean, even morally, he holds lower ground, he is literally the "bad guy". He acts like an ******* his first time seeing Viktor, for no reason at all. He doesn't have any idea why he is fighting either!
Viktor trained his whole life for that moment - he was raised in Ukraine his whole life, certainly a less comfortable place than the States, by his father (a boxing machine), both of them dedicating all their time towards this. Viktor literally started from the very bottom; he eats porridge for breakfast, and manual labour as his job.
Let's compare: Adonis is introduced in a juvie setting, but he is found by his mother, and literally spend most of his childhood in a mansion, with a mother, who knows nothing about boxing in compared to Viktor's father. He went on joyride fights in Mexico's underworld, but his scene in the boxing gym with Roy Wheeler, hints that he doesn't even have that much amateur experience! I mean, he has his life made, with what looks like a relatively high paying office job, while Vik is busting his backside in manual labour - even Vik's job constitutes to his training.
I think what irks me the most about this is not the result, but what the result shows - Adonis' talent won out against Viktor's lifetime of hardwork. Even with Viktor's low morale due to his gold-digging mother shouldn't have crushed that. As a person who has never showed any natural talent in anything, this pisses me off the most.