I've been making my own picture frames but the joints are off slightly so I
have to resort to filling the joints and painting.
What would be a good way of making these cuts so the miters would line up
tight enough to be left natural or stained?

I've tried a hand miter box, My 10" power miter saw, my table saw....

So what is the best way to get that cut at exactly 45 degrees.

TIA