What's generally considered the best (in terms of cost and compatibility) phone to flash a custom rom to? My current one is filled with bloatware that I can't uninstall and the bootloader is locked.

It depends upon the model of phone you're using. Can't you unlock it's bootloader? I mean I had the same issue with my phone as well but I unlocked it's bootloader.

I guess everything from Oneplus will have some of the best Community/Customrom Support and a somewhat low price.
The Pixel and Nexus devices also got good custom ROM support.
Besides that... Most flagships from manufacturers like Samsung and Huawai got pretty good support.

Don't get a cheap phone.
Get a old, used flagship instead.

Well, at least for bootloader unlocking, Xiaomi allows customers for doing that. They have a website where you request to unlock the bootloader of your phone and using a tool made by themselves, you do it. Very easy.

@ocalderon it's the same procedure like huawai does.
But this is _not_ easy.
You rely on some Website/tools from the manufacturer which might go down at any time.
In devices from Oneplus for example, you just reboot into fastboot, type unlock bootloader and you are done.

Don't. Mostly it breaks the camera and the GPS.

Just root it and uninstall the crapware afterwards. It's the best compromise.