So Mach 3 Turbo etc aren't worth looking at?I think the Mach3 is about as good as it gets for cartridge razors.
You can get very nice aftermarket handles for the Sensor/Sensor Excel from likes of Pils but the blade quality* is not as good in my experience. I can get a reasonably good shave out of a Sensor but it's not a pleasant experience - I find it harsher and less efficient at removing stubble.
The Contour is a pretty good razor and you can find aftermarket handles easily enough on eBay & Etsy. It was branded as the Atra in the US so a search for that should turn up plenty of options.
The Feather cartridges like the FII Neo are extremely sharp and smooth, cheaper than the Gilette carts but have a terrible tendency to get clogged. If you have heavy growth or like to wait more than a day or so between shaves I wouldn't even bother. The handle they come with looks like something from the Starship Enterprise but they do fit on the Gilette Contour/Atra handle.
*Among other things. I compared a M3 vs Sensor cartridges recently and as far as I can tell, the Sensor has the blades set at a more aggressive angle.