"Karl was a great boardgamer, if not the best in the village."

So what does that mean?

"Karl was a great boardgamer, but not the best in the village."
or
"Karl was a great boardgamer, perhaps even the best in the village."

They're both valid interpretations, even though they're opposites (in one I'm not the best; in the other I may be the best). "If not" is an inherently ambiguous construction - so avoid using it unless you intend to be ambiguous. Instead use alternative constructions which make your meaning clear. Readers will thank you for it.

Further reading:
P.S. I am quite good at boardgames.