Teen Titans, I have to say, was the best show out of all three. I think Hi Hi comes in second for me though because every time it came on, I'd stop whatever I was doing (or whatever I was watching, since it was on so early in the summer mornings) to watch it.
I think the reason I liked it so much though, like Spongebob, was that there were certain phrases and banter that only older teenagers and adults would understand, especially with "In The Cards".
In that particular episode, as most of y'all would know, Ami and Yumi compete in a battle of pseudo-Yu-Gi-Oh! for Chad (who I personally would've called an asshole - period.) The phrase, "... mack on his majesty," quoted from Ami, is what made me laugh. My nephew, who was (I believe) 5 at the time, didn't understand, of course.
The other reason I liked Hi Hi so much was because of the tidbit clips at the beginning and ending of each episode that showed Ami and Yumi horseplaying. Those were quite amusing. My favorite was the one where they talked about what city in Japan they came from. Also, unlike other shows, music exclusively by the artist themselves were featured in each episode, along with an animated music video every now and then. That was a stroke of genius, I think! Because of that show, I really got into Puffy and I love all their music. Besides, the animation was hip and very colorful.