If I want to do fantasy material is there a ruleset that is most popular with the OSR crowd?
Fuck popular.
What OSR ruleset do YOU prefer?
Do you want to write Generic OSR or OSR-line specific?
There's plenty of people who are happy with generic OSR offerings that they just modify to whatever they are running, but on the flip side, there are plenty of people who want supplements and adventures SPECIFIC to their favorite OSR ruleset. AKA, they want zero modification necessary when they play your stuff.
Whatever you do, make sure to PLAYTEST with actual people before you publish. The OSR - like the rest of everything - is 90% crap so if you plan to be in that Top 10%, your stuff better shine at the table.
Or have a huge art budget and sell recycled crap with pretty pictures. That usually does great on Kickstarter too.