I voted for 75%, but that's not really true. Actually 90-95% would be closer to the sucess rate I enjoy.
Your poll has a big problem, though: Either you have no idea of probabilities or your descriptions are completely off.
A success rate of 90% is basically a game that is _very challenging_! D&D (at least from 3e onward) generally assumes that it takes around 10 encounters to gain a level. IOW: at every level you faced an encounter that you didn't win. That's plenty!
In the 3e DMG there was a pretty good guideline for encounter difficulties that I used for my campaign:
10% easy (EL lower than party level)
20% "easy if handled properly" (EL 1-4 higher, unless the party does the 'right' thing)
50% challenging (EL equals Average Party Level)
15% very difficult (EL 1-4 higher)
5% overwhelming (EL 5+ higher)
In addition, it was assumed that a challenging encounter would drain 20% of the party's 'resources', so you could at most survive 5 challenging encounters before the party had to take a rest.
Given a party of five characters, that means there was a (very small) chance of a single character dying in a challenging encounter, namely if 5 times 20% = 100% of a single characters resources were drained, while everyone else didn't get so much as a scratch. Many of the official adventure modules made the mistake of only including 'challenging' encounters resulting in a 99+% of success overall.
Every increase in the Encounter Level (EL) should result in an additional 20% of party resources lost. That's why an overwhelming encounter is almost a guaranteed loss: every character will lose 100% of its resources - usually a TPK.
The risk of character death increases rather dramatically with each additional EL since resources are rarely depleted uniformly. With increasing monster levels, as really deadly abilities become more common, this becomes even more pronounced.
As my campaign has shown, character death became a common thing after the average party level reached 10. At level 13-15 (at which point the campaign was also finished), there would be 1-2 dead characters in almost every encounter. Luckily, raising the dead is no longer a big problem at such levels.
TL;DR: If 5% of the encounters are designed to be 'too hard to win', that's more than enough for the most challenging campaigns.
Having only a 25% chance to succeed in an encounter would make it impossible to run anything but a one-off session - it would be hard complete even a single adventure.
Having a 50% chance of success basically means that every encounter is a coin toss. Not many players would enjoy that. That degree of difficulty would make playing a meaningful campaign of any length impossible.
At 75% chance of success, campaigns can work, but they'll tend to be short.
Your poll has a big problem, though: Either you have no idea of probabilities or your descriptions are completely off.
A success rate of 90% is basically a game that is _very challenging_! D&D (at least from 3e onward) generally assumes that it takes around 10 encounters to gain a level. IOW: at every level you faced an encounter that you didn't win. That's plenty!
In the 3e DMG there was a pretty good guideline for encounter difficulties that I used for my campaign:
10% easy (EL lower than party level)
20% "easy if handled properly" (EL 1-4 higher, unless the party does the 'right' thing)
50% challenging (EL equals Average Party Level)
15% very difficult (EL 1-4 higher)
5% overwhelming (EL 5+ higher)
In addition, it was assumed that a challenging encounter would drain 20% of the party's 'resources', so you could at most survive 5 challenging encounters before the party had to take a rest.
Given a party of five characters, that means there was a (very small) chance of a single character dying in a challenging encounter, namely if 5 times 20% = 100% of a single characters resources were drained, while everyone else didn't get so much as a scratch. Many of the official adventure modules made the mistake of only including 'challenging' encounters resulting in a 99+% of success overall.
Every increase in the Encounter Level (EL) should result in an additional 20% of party resources lost. That's why an overwhelming encounter is almost a guaranteed loss: every character will lose 100% of its resources - usually a TPK.
The risk of character death increases rather dramatically with each additional EL since resources are rarely depleted uniformly. With increasing monster levels, as really deadly abilities become more common, this becomes even more pronounced.
As my campaign has shown, character death became a common thing after the average party level reached 10. At level 13-15 (at which point the campaign was also finished), there would be 1-2 dead characters in almost every encounter. Luckily, raising the dead is no longer a big problem at such levels.
TL;DR: If 5% of the encounters are designed to be 'too hard to win', that's more than enough for the most challenging campaigns.
Having only a 25% chance to succeed in an encounter would make it impossible to run anything but a one-off session - it would be hard complete even a single adventure.
Having a 50% chance of success basically means that every encounter is a coin toss. Not many players would enjoy that. That degree of difficulty would make playing a meaningful campaign of any length impossible.
At 75% chance of success, campaigns can work, but they'll tend to be short.