For the last few years, the only games my sons would play online were arcade games. I have to admit that these games are quite interesting, and if I was lucky enough to have some free time, which I did have some back then, I would sit with them while they played and get quite involved myself. Although I do not have as much time for that anymore, those will always be some of my favorite memories with my sons.
The thing about arcade games is that they are very intense. My sons would often get very worked up about the games, and that was despite most of the fun of it. The world inside that game is very real while you are playing it, and it would often seem like that was all there was to think about. Although arcade games are pretty common, there are only so many that can be considered very good. Everyone likes something different anyway, so there is something out there for everyone.
You do have to be careful when you play arcade games however, as they can be quite addictive. If you have any sort of work to do, you should do it before you play arcade games. My sons would get lost for hours in their online games, and there were a few times when my oldest son would almost forget to go to work. This can happen with any online game though. I know a few that I play have sucked away more time than I realized. It would seem the arcade games are the worst though.
If you are looking for some great arcade games to play, you might want to find some reviews first to see what you might want to try. You can find these all over the internet, and you can usually find tips and tricks to help you along if you have no idea what you are doing. Some of these games are more expensive, and it's hard to know what you will like and what just will not work for you. You might also try free online games with someone you know to save a little money on buying new games.