Secure is obviously a very wide term but as an approximation you can look at how much exploit vendors will pay for an exploit, assuming higher price = harder to exploit = more 'secure'.
I've worked on software for large pharma companies and several of our customers required escrow agreements. In our case it was to protect against us going under rather than just not wanting to do updates any more.
On that measure Chrome wins easily: https://zerodium.com/program.html