I would never post a poem I had written and say "that one's my best." My reaction to others who have done so is, why would you want to do that? If someone didn't happen to like it, they wouldn't have to bother with the rest of your opus. After all, if "that" is the best you can do... Maybe I'm thinking about it perversely. Maybe it's the depression talking. Not wanting to open myself up to derision at this particular point in time. I'd rather say, "here's a poem, of course I've written much better ones..." When in doubt, reach for the false modesty.