Absolutely loved Sonic the Hedgehog (SatAM) as a kid, mostly because of the great action and characters. Looking back more recently thanks to the series DVD release, my opinion dimmed somewhat. But I still consider it the best of the Sonic shows.
I've always considered Adventures of Sonic the Hedgehog as the show I didn't mind having around back when it was on, but never found anything special about it. Could be funny at times (arguably too cheesy others), but had its moments. Should really think about picking up the DVD releases to see how it's held up... did they ever end up putting all 66 episodes out on DVD?
Never saw Sonic Underground because it was always on too late in the morning in my area, but the bad things I've heard about the show have over the years kept me from seriously seeking episodes out. The biggest thing... to this day, I still couldn't wrap my head around the decision to have Jaleel White voice all three hedgehog siblings, including the sister! Oy...
Sonic the Movie... singlehandedly made Knuckles uncool to me 'til Sonic X came along. Then again, pretty much everything about the movie made the entire franchise uncool. The acting, the plot, the president's daughter... ugh, don't get me started on her! Maybe I'm lucky my VHS of the movie is buried in some closet somewhere in my house.
Sonic X screamed wasted potential to me. First two seasons adapted the first two Sonic Adventure games loosely. They did a decent job, but something about the show still seemed off. Or someone. Chris Thorndyke. Okay, maybe I didn't hate him as much as a lot of people did, but I didn't exactly like him, either. Was kinda there, needed to be saved every so often. I actually appreciated him growing up for Season 3, which made him likable (and by consequence, a whole lot less useless). Season 3's storyline was great, in theory, but once again, the execution left a good bit to be desired. How much 4Kids and their editing of the series affected the series is debatable, but for whatever reason, I could never fully get behind Sonic X at any point in the series' run.
Personally, I hope that if SEGA ever does another Sonic animated series, they follow a similar model to Sonic X. Keep all the things that worked, and change or remove those that didn't. Sadly, given the last few major Sonic games and their quality over the years, I'm not sure my childhood love for Sonic the Hedgehog could stand another mediocre (or worse) animated series.