In the olden days, you could mark your own reply as best answer, if you figured it out yourself.
This is no longer possible.
Why ?
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Good one 
In addition, I think it is not possible to change the best answer, if you misclicked or someone comes up with a better answer.
Ah yes, the game of cat and mouse each time the forum migrates… Some functions come, and some functions go…
I would suspect that being able to mark your own Answer as “Best” would unfortunately be susceptible to abuse.
For example, there are definitely some bots that have posted here in the last few weeks. Not many, but some. If bots were also allowed to be able to answer their own question as “Best” this could promote their content/profile and whatever spam/URL phishing that is typically within their posts, which is unfortunately today is very common on websites.
Also for any legitimate forum poster there may be a temptation to possibly “pad” their own Points stats, lets say for instance if it was seen that a certain cheating FME lizard was pulling ahead on the Points scoreboard, then maybe we could “fix” this and catch up by raising and answering a few of our own questions. 
I would suspect that being able to mark your own Answer as “Best” would unfortunately be susceptible to abuse.
A possible bot abuse if no reason to prohibit sensible usage by regular users. Bots should be handled elsewhere, and prohibited access at all.
And the best answer to a question to me is the one that solves the problem, regardless of who did it.
This way other users may find solutions faster, if they encounter similar issues.