What wire do I need when replacing with a higher gauge for a new cooktop?
Running 6/3 is future-resistant, but only a conduit is future proof

While many modern cooktops are designed for worldwide usage, running all controls and power electronics on 240V as that's what's guaranteed to be there no matter where you are, some North American market cooktops chintz out and rely on a neutral being present to run their controls, and the next owner may wish to rearrange things and put a range in, or tap an oven off this circuit for that matter -- both of these things typically require a neutral in North America.

So, running a 6/3 cable is more future-resistant than a 6/2; however, if you truly want a future-proof installation, you'd want to run a 3/4" conduit of some flavor to the cooktop location and then pull stranded THHNs through there -- ENT aka "smurf" is the cheapest option for this, just be mindful of the bending limits when installing it.  The conduit option also has the benefit of being able to run the wire at 75&deg;C, which allows you to run 65A over 6AWG copper.  This is plenty of current, even for the largest cooking appliance setups.