For me, this is a bit like asking an electrician what is his best tool? You can’t do the job with only one tool though, you have to pick the right tool for each application. There’s no generic “best” that can do all of it.
He might have a favorite tool though, one that has sentimental value maybe, or that he always uses along with the others, an old and familiar friend that has never let him down.
My old and familiar go-to banishing is smudging with white-gold heart-light. I posted how I do it here:
My basic go-to ward is this kind of shield here - everyone should just have one of these around them as a standard practice imo:
Ok, so…
There’s no such thing as an unbreakable circle, so you don’t have to beat yourself up about that.
Be clear whether you are talking about humans or astral beings, they are not the same and don’t behave the same way, or have the same tools and abilities.
Humans have free will, and that includes the ability to go “oh, show me what’s behind this barrier” and poof, you’re behind the barrier. Your circle is meant to keep out astral wildlife, parasites, casual opportunists and other randoms, they will never hold off a determined human.
You best bet for that is to go stealth - they can’t try to cross a line that they do not know exists, cannot find, or are distracted from accessing.
Witch wars are a battle of wits more than anything, though persistence helps.
Dopamine training to enjoy doing the hard things.
Pick a workout routine and get ripped. Make a new healthy protocol and make it a habit - the same ability that lets you do that is your will. And a strong body makes a strong mind, as you are a gestalt being and these are connected… Twofer! 
This guy explains it really well: