In no particular order:
Batman: the animated series / Batman Beyond
Seriously, you get a nice dose of action in every little episode, Batman beyond is lodged in as a sequel that keeps well to the spirit of the original with a sci-fi twist.... oh yeah... and Mark Hamill as The Joker
Spider-man (1994-ish)
Same reason as above, really does get a saturday morning going
Futurama
Really takes everything good out of sci-fi and just laughs about it...
Pokemon
Back when there were only 151, later on it just kinda repeated itself over and over again with new monsters...
Digimon
1st, 2nd and 3rd series, the 4th and 5th are a bit "meh" IMO
Sonic SatAM
as mentioned; too bad it got cancelled, it really needs a DVD release (region 2 also pl0x), but until then we at least have a torrent that circulates once in a while and the comics. Best voice for Robotnik too IMO (Jim Cummings)
MLP:FIM
Yeah, I got curious and joined the club. It's rare in this day and age that you get impressive animation mixed with good fun and great characters
South Park
*insert numerous positive f****ing cursewords here*, also it's hilarious to make out the words that Kenny is saying
I cannot for the life of me understand why i've never seen this on here...
DUCK TALES
for crying out loud IT'S FRIGGIN' DUCK TALES!! !!!1111oneoneeleven
Hi there got to agree with you on Sonic Satam however it has had dvd releases.I have the region 2 uk version which you can from here