Sonic Generations originally released the week before Skyrim, and a month after Dark Souls, so unless you're a massive fan of the blue creature you'd be forgiven for passing it over. It's actually one of the better modern Sonic games: it takes a hybrid approach, with levels evenly split between traditional sidescroller platforming and high speed third-person rollercoasters.
Anyway, it's currently a measly $1 (£0.75 / AU$1.20) on Steam (opens in new tab), so if you've ever felt inclined to give it a go, now's a good time. The deal is actually for the Sonic Generations Collection, which includes the Casino Night DLC which, as the name implies, takes place in a casino-themed world reminiscent of Sonic 2.
If that's not enough Sonic for you, most other Sonic games are going cheap too. I'd especially recommend Sonic and All-Stars Racing Transformed (opens in new tab), which is currently 75 percent off. The recent 16-bit styled Sonic Mania is also 66 percent off. Aside from those, I'm not sure there are any other good Sonic games. I'm pretty sure there aren't.