It's dependent on many factors. Answering under the assumption that you are talking about the U.S., then the points of your specific question - "most common" and "widely accepted" - mostly rule out World Standardized Rules (WSR).
In organized play in the U.S., the ACS is the only organization with any significant numbers using WSR, as far as I know. Around 22,000 according to their website.
Organizations and others not using WSR and a rough guesstimate of their numbers. Some numbers could be wrong - I rarely concern myself with such matters. Anyone out there please feel free to correct them.
- APA - ~250,000. About as far from WSR as you can get.
- BCAPL/USAPL - ~65,000. The closest to WSR, but with a few significant differences. As far as detail and clarity, by far the most comprehensive and understandable rule set.
- TAP - ???... "...tens of thousands..." says their website. Many significant differneces from WSR.
- NAPA - ~15,000? Many significant differences from WSR.
- UPA - ???...Operating in 18 states per their web site. Many significant differences from WSR.
- The "Any Room You Might Walk into in the Entire Country on Any Given Night" league. - Millions. At least as many different rule sets as there rooms, the HUGE majority of which are going to be playing with large differences from WSR, especially on Tuesday afternoons when it is raining.
The same goes for many professional events as well. A brief list of events that do not use WSR:
- US Open 10-Ball, US Open 14.1, US Open One Pocket, USBTC - all use BCAPL rules.
- Derby City - hell and gone from WSR.
- WPBA - when it was significant, significant differences from WSR.
The whole point being, WSR may "S" elsewhere, but not in the U.S. I'm not making any editorial analyses or judgments. Just telling it like it is.
Do your research, make your decision, and the best of shooting and luck to you.
Buddy