The best one I've ever used was a 4m Browning one that I had 20 years ago. Unfortunately I left it at a lake a long time ago and never saw it again. They're not made any more and for the life of me I can't remember the name of it, but I've not seen one pop up on eBay, despite looking.
So that doesn't help...
Not so long ago there was a guy who made them bespoke up to 6 or 7 metres in various strengths and stiffness, but fairly big money. Also looks like he's no longer in business as a Google search comes up empty.
So that doesn't help either...
Of the other options, the best I've tried is the Fox Matrix Ethos 4.5m, but at £120 it's not cheap. It is, however, made with larger fish in mind, unlike most other long handles which are more suited to commercial fisheries and match fishing. It's the one I'll be getting when I can afford it.
I'm currently using an el cheapo 5m Lineaffe to reach parts of the rivers I fish that a handle any shorter won't reach. It's not brilliant, but it's stiffer and a bit more rugged than the other 5m handles available from Preston. BUT it's still quite floppy and you have to be a bit ginger bringing it back in with a fish - feeding it back behind you pole style, rather than lifting up like a rod. I can't see it lasting long, mind. Get what you pay for and all that.