This decade was an odd one.
I have mixed feelings on Ben 10. One of those shows that got better as the seasons wore on, and one of the few popular shows during CN's 'dark originals' period. But Ben and Gwen could be off-putting at times... sometimes both at the same time. Honestly, though probably deserves a place... unless Ben 10: Alien Force is eligible, in which case I'd vote in the second series in as an overall better product.
Secret Saturdays, while exploring new ground with a retro feel, I felt really did not start to really wow me until late in the first season. I'm not really sure if that's enough to merit a Top 5 of the decade. Codename: Kids Next Door is probably a better choice... even though I had a falling out from the show in Season 4 and 5 and then loved the show again in Season 6 + finale movie.
Megas XLR is a touch cookie to figure out. I loved the parodies and such, but really, I can't seem to remember anything unforgettably memorable from it. But given some of the other options available, maybe no one else earned a spot in the Top 5, putting Megas in by default.
I started to fade away from Ed, Edd n' Eddy in the fourth season, but I don't think it was anything serious until they moved out of the summer environment and into fall for Season 5. New territory, yes, but the episodes just weren't that interesting anymore. Again, the finale movie made me love the Eds again, saving their reputation.
Samurai Jack... a great show, stands out for many reasons because of the whole 'actions speak louder than words' theme Jack had going. Great stories, great visuals, but again, as the series wore on, I had to wonder... I'm running out of things to say I like about this show. And the fact that there was no ending (Jack never defeated Aku or returned home) has to be a huge gash.
Foster's deserves the #1 spot, but, yes, again, the show fell off in Season 4 (wow, Season 4's the fall-out point for a lot of shows, isn't it?), but recovered towards the end of Season 5, had a good enough Season 6, and the finale was something special.
Wow... now that I look back, Cartoon Network never seemed to have a long-lasting show that was great all the way through. A good number started off well, fell off, but recovered to end on good notes, but still... and remember, these are the good shows! That doesn't put much stock in the total quality of the bad shows, does it?