I have first looked at other questions/answers here on stopper knots here and tried all of the ones suggested but I am seeking something better. My application is a single line through a hole. I want to be able to tension the line, so the act of tightening the knot does not create slack through the hole. I don't have slack through the hole before tying the knot. It is really not needing to hold any more tension that just pulling the rope taunt. I would also like easy untying. It seems a single overhand is maybe the best I can do thus far. Probably with a loop back (like how you tie your shoes) for even easier untying. This is a photo of it:
I'd suggest a "slippery figure of eight" - although i'm not sure if that's the right canonical name for it.
It's my go to knot I use when trying to shorten guy lines on a tent or tarp - i.e. when I have a runner with a hole and can pull some of the line through, tie a stopper knot and then use the shortened guy line.
When taking the tent down, a brief pull of the loose end and the knot is undone despite the tension it was under in use. And as a bonus, this knot can be tied in the middle of a line without needing access to the end of the rope.
How about a peg that fits snugly into the hole with the rope--you pull tension on the rope and capture it with the peg, adding a half-hitch or slippery half hitch to lock it off.
Or an Italian (Munter) Hitch (ABoK #1818) around a peg, locked off with a cleat hitch as needed:
I use this on a through-hole on an old sailing dingy with a missing stay. It won't slip, won't pull through the hole, and will remain easy to untie, even under load.
You are looking for the barrel knot: https://www.youtube.com/watch?v=yRfjF5TcQbw. This is used by climbers to make sure the rope can't pass through the hole in a belay device.