What element out of: Acid, Fire, Cold, Electricity is resisted/immune least often amongst monsters?
If it makes a difference, PFS games.
Most resisted = Fire
Least resisted = Acid
In general, I believe it goes Fire>Cold>Elec>Acid.
That's off the top of my head and the middle 2 could be switched, but I'm fairly sure that's the right order. I'm positive about fire and acid being most/least resisted though.
Of course, it WILL depend on the game/setting you're in. If you're going up against lots of oozes and whatnot, acid resistance will be common.
|The black raven|
I did an analysis of the 1st Bestiary some time ago, taking into account Resistance, Immunity, Regeneration and Vulnerability.
According to the weight I gave to these factors, the worst element is Cold, then Electricity. Acid is best but Fire is just behind due to beasties having Regeneration or Vulnerability (Fire).
Once you chose Acid, know that the next best element to deal with those creatures resistant or immune to Acid is Electricity.
Of course, including creatures from other sources might change the results.
It might be worth considering not the entire breadth of the Bestiaries but only that subset which one might encounter in one's campaign. Even if you're wondering about the "overall" picture, you're far less likely to fight an angel, for example, unless you're an evil character. What is probably more relevant would be the resistances of demons. That said, a DM can always put together a badguy with a lot of acid resistance if he wants to.
Also, don't forget about "sonic" and "untyped".
Here's a spreadsheet with all the compiled statistics from the bestiary by Shoelessinsight.
Resistances are found on the third tab, and are broken down by level.
It looks to me like they vary wildly with level, but are all about even (except sonic of course, which has hardly anything).
Of course, this is completely ignoring the choices made by adventure designers and DMs. They might very well choose fire resistant enemies most of the time.