Can't say for sure what's the "best" - but I'll share my experience.
I bought a Dowco Guardian Weatherall Plus for my INT 650. Unfortunately for the time being mine is outdoors 365 days a year, so being weather-proof was my main concern. The construction seems very solid and so far it has held up through several hail storms. Much more weather-resistant than any other cover I've ever used. But it should be...it wasn't cheap.
Only a couple of complaints: For one, I wish I would have bought a Large instead of the recommended Medium. It really just barely fit with the stock mirrors, but fits better now that I've put lower profile mirrors. The other issue is that I bought the black version, and it really isn't as highly visible in the dark as I'd like. It has a 6 inch reflective strip that isn't in the best spot. I park on a somewhat busy street where people get desperate for a parking spot at night. I'm increasingly worried that someone will mistake it for an open spot and zip in and smash it...so it looks like I'll be shelling out another $10 or so for some hi-viz reflective tape. In retrospect simply buying a lighter color would have been smart.