¿Qué onda? definitely feels the most informal of these to me, and so What's up? or something more slangy like How's it hangin'? is how I tend to translate it in my mind. (And the sometimes (and even less formal) pronunciation ¿Quiooonda? turns into Wazzuuup? in my mind.)
I'd additionally say that ¿Qué pasa? can tend to have a connotation of something being wrong, so can sometimes be interpreted as What's the matter? or What's wrong?, usually in a caring sense, but sometimes even aggressively/angrily. For example, depending on the tone of voice, ¿Qué te pasa? could even mean What [the #&@$] is the matter with you? (though I realize the "te" here makes it a bit of a different case). I'd imagine that a lot of where ¿Qué pasa? falls between a neutral _What's going on?" and a negative What's wrong? probably lies in tone of voice or other context, but there could also be differences in regional usage at play that I might not be aware of (I have most experience with Mexican, then Chilean, then northern Central American speakers).
If you're looking for something of a "go-to" option among these choices, I'd agree with Danielillo that ¿Qué tal? works well as a relatively general/neutral/widely-used choice. I'll note though that if you mentally translate it in your mind as What's up? (as I did when first learning Spanish), it can be confusing to hear the typical response of bien (well/good), since as Danielillo (and some responses in the link in your question) mention, I believe it's a shortened form of a longer unstated expression. So I'd agree with him that How's it going? (or How did it go? as the context might dictate) would be a better way of thinking about it.
I'll also throw in an additional variation that I hear less than the above (and I guess is technically a past tense variation of ¿Qué hay?): ¿Qué hubo?. It feels informal/slangy to me, especially when used in variations Qué húbole? or ¿Quihúbole? (the latter feeling similar to me to the aforementioned ¿Quionda?).
Since I'm not a native Spanish speaker though, I'd be happy to be corrected where any of my above interpretations or nuances might have missed the mark.