N1EE wrote:

What's the best cotter pin type to secure
a turnbuckle?

I like to use an oversized ring type cotter pin. They still have to be
taped over (it's a commonly heard myth that cotter rings won't snag
lines) but they are easier to use and have longer useful life. Some
people I sail with use safety-pin style cotter pins. These are fine
but I think they are more obtrusive.

Fresh Breezes- Doug King