Assetto Corsa - very nice, great graphics, great sound, great physics and its still only a beta, lots of great things to come from this title however there is currently no multiplayer (someone correct me if I am wrong here but I don't think there is currently) but if you like racing alone or hot lapping this is a dead cert.
Euro Truck Simulator 2 - fantastic driving simulation not racing, please note that, and the MOD community for this game is amazing some truely fantastic stuff out there to really challenge you in this game as you build up your business empire and most of it is free to download as well.
iRacing - very good all round racing simulator however this has a monthly subscription as you are aware, if you are willing to put in the time and race seriously then this is a good shout, this won't really work as a casual racing game you need to put some time into this to get the best out of it, thats what I found in my experience.
rFactor 2 - Hmmmm still not sure about this, the graphics engine is somewhat lacking a modern feel, sometimes it looks great others it doesn't, MODS are slowing becoming more availalbe however not as readily avaialbe as they were/are for rFactor which is still a pretty darn good racing game and has a massive community who still race this online. Physics on this game are very good, its had quite a few patches lately and does seem to be getting better bit by bit.
Richard Burns Rally - great game, it has some amazing MODS out there and the community for this somewhat 'ageing' title is still very much alive, definately one to look at - I'm still amazed this is still going strong.
F1 2010/11/12/13 - If you are going to go for any of them F1 2013 is the one to go for not just because it's the most recent release it's also the most 'sorted' there are lots of MOD's available for this and if you get the classic contents is adds a nice historic element to what already is a very good game.
I have all of the above and do play all of them from time to time, they each offer different aspects of racing/driving however all are great fun, iRacing is the only 'odd' one in the pack as it is more of a 'serious' racing simulation than game, the rest you can pick up and play as and when you please really.
Stu