Digital Ocean. I have directions for TinyMUX around here somewhere. If you want Rhost it's similar-to-identical and @Ashen-Shugar is also insanely helpful, as well as anyone on the Rhost Support Mush.
Evennia's setup is also stupid easy. That's the end of its stupid-easy, but hey, options!
For codebase: This depends 100% on your coder. I'm going to suggest Rhost over TinyMUX, but all of my code is TinyMUX so if you're starting there you're kind of stuck.
Ares if you're using FS3 and love the Portal/Scene concept. There's a lot there to love, but be sure it's the system you want beforehand.
Evennia if you're starting from 100% scratch and want to build a skyscraper taller than anything we've ever seen.