I am currently running interim release 9.6(4)42. I want to upgrade one last time before I cannot get contract support next November. Can anyone recommend a good major release that is stable. We use Firepower 184.108.40.206 with an FMC. We also run Anyconnect 4.7 with about 125 users. Highest concurrent user for vpn has been about 85.
When there's a Gold Star version in the downloads page go with that. For AnyConnect Cisco generally recommends (an I agree) to go with the latest version.
So in your case that means:
(need to upgrade FMC to that version first)
Unless you have specific needs that might be catered only on certain releases, which is very rare, always go with the version that have the gold star next to them.
If there are more that one gold star versions, then see which version does what you need exactly.