I want Baalzebul (from the 3.5e Book of Vile Darkness) to be a behind the scenes villain in a custom adventure I'm preparing. There will be a priest that's worshiping him.
Is there a 5E PHB deity that directly opposes Baalzebul?
I cannot think of any deities specifically called out as opponents, but we can do the next best thing by extrapolating from other background details.
Baalzebul was once a Celestial Archon, presumably in service to a good deity. Presumably he was not a lantern or hound archon, but one of the other types as he is noted as having had an "angelic" visage. Deity Do's And Don'ts by Sean Reynolds notes that archons of these other kinds were found in service of Arvoreen (Halfling god of defense), Berronar Truesilver (Dwarven god of family), and Deneir (art, knowledge, and scholars). Of these only Deneir is in the 5e PHB and is also the most open for any PC to worship.
Deneir's alignment is NG and has the Knowledge domain, and is a patron of those who pursue learning and self-improvement, which fits nicely when considering whether Baalzebul could have been a former servant, as the archdevil had a noted ambition and constantly strove for perfection (before he was cursed by Asmodeus with his new form).
While this is by no stretch of the imagination a canonical interpretation, it is plausible that Baalzebul was once an archon in service to Deneir before being cast down to the Nine Hells. Whatever motivated such a fall in the first place could easily be the start of an eons-long conflict between the two and it would be very easy for you to build from that into a fully-fledged rivalry.