The most balanced bus simulator
OMSI Bus simulator is so far the most balanced bus simulator I've played. Graphics are reasonable to it's genre, the driving cockpits are quite good, most buttons and lights have a reason to exist, work and actually makes something inside game. You can control a lot of things in your bus and some of them your passengers will want you to do so. Control the inside lights, temperatures, windows, you can literally do everything you would do in a real bus, including selling tickets and control the IBIS system.
Sound is very good, for those who actually been inside such buses you will find it very familiar. You can in some models use the tape player, it does not open music files of your computer but works with online radio streaming of your choice, which is very nice. Unfortunately it lacks a radio volume control so you might not listen to your bus sometimes...
Overall physics are reasonable, could have some improvements but are in fact the best of it's kind.
It has many other features, I encourage you to try if you like somewhat "professional" simulators. It also has easier (but not so realist) ways to put you driving, so I considered it's has de ideal difficulty: can be easy for those who don't get it or more complex to those who are in fact familiar with buses.
It has a good modding community and hopefully will feature more buses (lacks articulated buses, compact buses, one deck buses and also tourism coaches). You can already download many of these content made by modders as well as new maps.
If you like buses, make sure you give it a try ;)