My favorite is honestly
Sonic SatAM. That show alone is what started the Archie Sonic Comics series with the freedom fighters. I recommend watching that one as your first Sonic show.
The other 2 Sonic shows, with
Sonic Underground and T
he Adventures of Sonic the Hedgehog, I recommend watching Underground after Sonic SatAM, then The Adventures of Sonci the Hedgehog.
Sonic Underground is kind of a different alternative timeline from
Sonic SatAM.
I was actually re-watching it the other day and I find it to be very cheesy and boring at some episodes, but if you're a die hard 90's Sonic fan, you'll dig it. Especially the 90s slang and rock band style.
The Adventures of Sonic the Hedgehog is the last of the 90s Sonic TV shows, I recommend for someone to watch. Because the cheesiness surpasses Underground's. In fact, think of Looney Toons but with Sonic, Tails and Dr.Robotnik (Eggman) instead. And there you have it.
Now be warned, there are a lot of gay undertones in this cartoon. Just like how Bug's Bunny crossdresses to trick Elmer Fudd, the same with Sonic crossdresses to trick Dr.Robotnik in some episodes.
Some can handle it, but others find it too much of a cringe, especially if they didn't watch this as a kid. To me, it's a guilty pleasure because it's honestly funny. This is like the slap stick comedy version of Sonic.
Sonic X and Sonic Boom are the very last you should watch. Because Sonic X was honestly boring and wasn't as fleshed out compared to the three 90s Sonic TV shows. You will only understand this if you start from the beginning and work your way up and you'll be dissapointed with the lack of characters and classic Sonic comedy.
That's pretty much my take on the order to go with the TV series.