Ultron (and his clones) vs Steppenwolf (and his parademons)
Who is taking this? Steppenwolf and Ultron get 20 parademons and clones a piece. I'm talking about the standard clones for Ultron. The ones that fought against the Avengers at the end with Ultron.
Stepp was able to dominate the League minus Superman but Superman outclassed him big time.
Ultron on the other hand took it to the Avengers but he was beaten down by Hulk. Thor, Vis and Tony were able to heavily damage him but there was an Infinity Stone involved... on the other hand even Cap had a decent showing against Ultron in combat.
Step seems to be able to cause more widespread damage with his axe than anything Ultron was able to do. Also Parademons are probably superior 1v1 to Ultron bots if we look are their low showings Widow and Hawkeye were able to destroy the bots with little trouble while Batman had some trouble doing the same to Parademons; I know ABC logic isn't the best comparison but it'll have to do here.
In a group battle though the Ultron bots would prove superior due to being robots (no pain receptors) and of a hive mind. Both armies have decent swarm though.
I dunno its pretty close but end of the day id say that Steppenwolf could take Ultron 1v1 but if they both had their armies Ultrons would come out on top in a good battle.