What do you think was the best year of CN?

I like almost all eras of CN including now, but 2005 was my favorite year. It's like a line graph where it hits it peak in 2005, and then starts going downward fast (before picking up again in about 2010). 2005 was when cartoon cartoons and shows being made around that time were balanced pretty evenly. 2005 was also a great year for Toonami, with TOM 3 and better western programming like Ben 10, Teen Titans, and Justice League, while still showing good anime, though it was mostly shonen.

1997 - 2000 HB, Britt Alcraft, D'con, WB and lots more era. Or if the answer can only be a CN era it has to be 2000 - 2001 when it was still mainly HB and WB classics but new Cartoon Network made episodes of former HB propities were being shown too which makes it a changeover era but still it's the best if I have to choose a Cartoon Network era only as it was still HB cartoons but changed over to the new CN Made brand for the later seasons at that time.

Forgive me in advance, as I'm not great with dates, but my personal favorite era of Cartoon Network was from around 1996-97 to around 2000-03, basically the range from the Powerhouse and City eras, when Space Ghost: Coast to Coast, the Cartoon-Cartoons, Toonami and Miguzi were around.

That era had a nice sufficient mix and variety of programs: there were first-run comedies as well as action cartoons, some decent acquisitions (Freakazoid!, anyone?), the network wasn't afraid to have shows with female protagonists (CN didn't have a big 'NO GIRLS ALLOWED' sign plastered on their walls like today), Adult Swim had just been launched around that time, but it was actually good; it was only on 2 nights a week, the shows were actually entertaining (the aforementioned SG:C2C, Home Movies, Mission Hill, Sealab 2021, early Aqua Teen Hunger Force, Cowboy Bebop, the only dud in that lineup was The Brak Show, IMO), and this was before the FOX animated sitcom imports came along and began taking over the air space. Good times.

My least favorite era of CN was the Checkerboard era (roughly between 1992-1995), due to the network's extreme lack of variety during that time. The CN of that era was basically the Hanna-Barbera Reruns Channel. Some of the old-school HB shows are OK in small doses, but 90% 60's through 80's HB shows from cover to cover gets old real fast for me.

CN of today is OK. I wish they'd get off this 'dude/bro' "boys are the only demographic that matters to us" kick they've been on for the past few years, as that puts a definite limit on the types of shows they green light, as the channel could use some diversity, and I'd really like to see Adult Swim's presence trimmed down, but it's more watchable now than it's been in years.

New CN has grown on me now with the invention of CN OD on the UK SKY box I must admit although there is some poop ones like the new Transformers where it's 3D on a TV animation which is rubbish and rips off 3 characters from movies and other shows and those 3 humans drive the transformers when they are supposted to drive by themselves what utter rubbish.

But shows like Mordicai and Rigby's Reguar Show, Jake and Finn's Adventure Time, Gumball, Ben 10 (Normal), Looney Tunes, Pink Panther, Johnny Test ect. are ace.

I don't count Pokemon though as thats on CITV and Disney XD for us.

I'm going to separate CN and [adult swim] for this reply! because I do consider them separate entities.
To me the 90s in general were the second "golden age" of cartoons (the first being the 1940s). This time was began with the boom of Nickelodeon in the early 90s, with classics like Ren and Stimpy & Rugrats. However CN took over the reigns in the later 90s, and that's where the high point of CN began. If I had to pinpoint the years, I would say that the best for CN was from 1997 to 2007, about a ten year span. They gave us some of the greatest cartoons ever (Powerpuff Girls, Johnny Bravo) and some of the most appealing, yet darker cartoons ever (Courage the Cowardly Dog, Billy and Mandy). It was a time of brilliance.

However around 2007 most of their best shows were already gone or on the way out. Although some new shows at the time (Chowder) could measure up, as a whole the network started down a slippery slope. Today the network is in ruins, but cartoons in general are going through a rough patch. I can only think of a few CN cartoons I actually like, the rest is just dreadful, uninteresting garbage (Regular Show)
So, to sum up your question, I would say 1997 to 2007 was the best time for CN. I sincerely hope they - and cartoons in general - can rise back to a higher point. Until then I guess anime will have to do.