Sonic X has got everything. Great characters, excellent storylines and the pertentional for sequals. It is by far the best Sonic cartoon series. Plus, it also has every character from every Sonic game, including Espio and team Chaotix. Plus, in the last episode of the 2nd series, we get to see Super Sonic, who totally destroys a Robotnic robot with 'Chaos Control'. I just love it.
Shadow does appear, but it's near the end of the whole series. If that's not enough, he appears because the series does a adaptation of Sonic Adventure 2. It even includes an appearance of Super Shadow. I also believe that it follows Sonic Adventure 2 to the letter, except Rose asks Shadow to help Chris, not Sonic.