The non-trinitarian heresies are interesting because they're terrible, but they're terrible in different ways.
Arianism is the most theologically problematic, because in Orthodoxy (and to a lesser extent in other branches) the point of Christianity is the union of humanity and divinity through the person of Christ, so if Christ lacks divinity or humanity than Christianity's whole purpose falls apart.
Modalism is the most biblically problematic, because the relationship between Christ and His Father is a major theme of almost every book of the New Testament, and so denying their separate personhood turns darn near every page of the New Testament into exegetical gymnastics.
Tritheism is the most philosophically problematic, because having three gods necessitates denying that God is the Ultimate Origin of All Things (since such a point is by definition singular), in the process turning Him into a being in the cosmos and raising the question of why He's worthy of worship or philosophically necessary.
you could also argue that Tritheism is biblically problematic because God claims to be One repeatedly in both the Old and New Testaments. He's quite emphatic on that point in fact.
there's a reason why the standard explanation of the Trinity (or non-explanation?) is the one that has stuck around the longest, because any attempt to "explain it away" in any way that fits into our happy little human-made boxes results in nonsensical contradictions.
at some point as a species we have to just accept that God, being the most unique Entity in and out of existence, simply does not function in ways that make sense to our tiny little brains and tiny little experiences. This is something we face in physics regularly (quantum mechanics notably has so many things like this built into it's math that whenever someone asks too many questions about what something is or how it works the standard reply is "SHUT UP AND CALCULATE"), why not faith?
If we believe in New Testament, we know that the Trinity and Incarnation are real because there is no other explanation for what we see in scripture (or in Eucharistic miracles for that matter).
you could also argue that Tritheism is biblically problematic because God claims to be One repeatedly in both the Old and New Testaments. He's quite emphatic on that point in fact.
When I said modalism was the most biblically problematic, I was thinking on a more structural level. It's possible to write a summary of, say, St. John's Gospel or St. Paul's epistle to the Galatians, without mentioning that God is One. Only someone with a strong prior commitment to modalism could summarise them without mentioning the separate personhood of the Father and the Son.

















