What alternative OS you adopt depends on your phone and needs.

If you just want to mostly degoogle you phone, but aren’t that concerned about privacy or security, LineageOS is basically that. It’s as close to a stock android experience as you can get, minus the pre-installed google apps. It supports a wide range of android phones.

If you want more security and privacy, GrapheneOS is currently king, but the downside is it only works on Google Pixel phones, and it’s possible certain apps won’t work (banking apps can be hit or miss, and the Uber app I believe blocked it). It otherwise functions just like a regular android phone and can install sandboxed google play to use any app you could need, making it just as easy to daily drive. It’s the best choice if you’re an activist, journalist, or fear state actors. But even if you’re not, I’d say it’s the best choice if you already have a pixel phone.

CalyxOS has paused development, so not currently an option.

eOS has a bigger focus on security and privacy than LineageOS, but isn’t as secure as Graphene. The advantage is that it supports more phones than just the google pixel.

PostmarketOS is not based on Android, and instead is a real Linux Distro made for mobile. It’s still very much in an alpha stage, with varying levels of support for different phones, many of which cannot take calls or even use their camera properly. I would only recommend it to developers or people who want to tinker with a project phone.