Sonic is celebrating his anniversary with style! You can go on Humble Bundle to get a bunch of Sonic titles across the series on Steam and the more you pay, the more games you get. Here is a link to the page: https://www.humblebundle.com/sonic-25th-anniversary-bundle?view=xMUV9Ar1PHw_
The following games are a part of each tier:
1$: Sonic 1, Sonic 3D Blast, Sonic Adventure DX, Sonic CD HD (2011)
10$ or More: