A good way to go if you aren't sold on a whole pack, is to buy 1 car from the class. You can race online and get all the class cars in your SP races. DTM 2015 is very good as are WTCC 2014/15. The Hillclimb pack has a good variety of cars in different classes. I like the Group 5 and GTO's but they aren't easy. The F2 is awesome as well. Bathurst, Spa, Nordscheife, Sonoma are must haves, IMO. The track packs are good value though ( Euro or American).
Quick pick: Hillclimb pack and Bathurst.