For 240V at an RV park, I would think that the most popular is what they call "50 amp service", which is delivered with a 14-50. I got a Camco 50 amp RV plug and removed the neutral blade when I made an adapter. Now it fits both 14-50 and 14-30 sockets. In homes, you're likely to see either 14-30 or 10-30 dryer outlets, so a 10-30 adapter might also be useful. So far as other places, it really depends on what application the 240 volt outlet was installed for... Probably 6-20, 6-30 and 6-50 are popular for various applications, as well as the locking versions L6-20 (no adapter needed) and L6-30. I've seen L14-20 and L14-30 used on some portable generators.