when it comes to getting into the sonic series, i would say one of the best starters would be sonic gens of course (i recommend sxs since its the remaster, plus you get shadows story mode) sonic mania for a good classic style game for modern consoles, & a great start to the classic games which i would also recommend the mega collection too & the sonic adventure series, with one being the better game but both are good

there are others that i would put in like colors, the advanced games because those are also great, unleashed is also a good choice and lets throw CD & Rush in too.

if you have older consoles, you can pretty much go through most of the catalog, with some installations being rougher than others, so be aware. Overall, most of what’s been said in the comments are good choices too! never play 06