It's kind of funny to me that so many people are picking such extremely low numbers, because that results in terrible gameplay in practice, because it explains why spells, which don't need to make a check, just arbitrarily do something, are so much more powerful than skills. Honestly, anyone picking 40% or below, you're part of why spellcasters are so dominant in the two non-combat pillars, especially at level 5+. The idea that someone "good" should just fail 60% of the time, whereas a spell should fail 0% of the time is deeply problematic, design-wise.
Also, if spells had a similar fail rate to skills, which they probably should, I bet we'd see very different numbers being picked by the same people.
I picked 65% and 70% which are, I see also pretty popular (interesting), based on my experience with RPGs. If you invest in something, to the point where you're good, and still fail the majority of the time, the message your giving the player is "your decisions were worthless". Again especially in D&D where spells and many abilities just work 100% of the time.