Which Sonic game would you recommend to a newcomer?

I don't think that's entirely true, I've met young players who like pixel art. I myself played Classic Collection on the DS when I was younger. As long a retro game collections are around, or indie games that use pixel art, there's still going to be a group of players who like games like Mania.

But yeah. I guess your average younger player may not like it. (My younger brother's friends think Sonic Mania looks terrible)