If you're just gonna play one, get Sonic CD.

There are several reasons I recommend this game. First, it's cheap as hell during this sale. Second, it has the best music in the series (thanks to its optical media). Third, the level design is great, with multiple layouts to each stage based on time travel. Fourth, it doesn't have the story or overabundance of characters that weighs down more recent Sonic titles.

But that absolute number one reason I recommend this one is because it runs on Christian Whitehead's RetroEngine. This is an amazing project by this guy that allows Sonic games to play in 1080p @ 60 fps with no drops at all. It's a beautiful way to update old games, because it removes artifacting, takes advantage of widescreen and retains the pixellated charm of the originals. He updated Sonic 1 and Sonic 2 with this engine as well (though these ports are only on iOS, don't buy the Steam versions of these, they're shitty rom dumps), Sonic CD, and he's applying it to the upcoming Sonic Mania.