I see two questions here. First, the question in your body.
I want to be able to focus on just 1 story, and actually finish it. Any advice?
Go until you get distracted, then move to the next idea and circle around again. This is rather common, and is what I do. Switching between stories keeps you writing, and if something doesn't quite seem like it fits in one story, toss it in another.
I don't think sticking with one until it's finished is a good idea, because when you have another idea, you'll rush to finish so you can move to the next, and then you end up with a very abrupt and probably also low quality ending. Not that abrupt endings are that bad, per se, but there's a time and place for everything.
And then the title question.
How to find out which story idea is the best to write & too many ideas. Advice?
There is no set scale of how good a story is, as there is just too many factors that go into it. Cover art*, writing style, plot, character relations, character personalities, character interactions, etc etc. Worldbuilding alone is so important it's one of the larger SE sites.
When choosing what to write, just pick what you feel like writing and write it. Don't force yourself to write, as that is one sure way to produce sub-par chapters.
*- you could argue this isn't a factor in writing, but it is in publishing which is why I've added it.