In one word, Prince of Tennis would be "exhilarating".
Tl;dr at the bottom.
There are many flaws—the lacking animation and art, for one—but they can't overshadow the essence of this story.
You can complain about the unrealisticness of everything, about their 'superpower' tennis, about everything that makes this so ridiculous. However, in the end, I will always say this: Prince of Tennis is a masterpiece unlike many, or any, others, and I don't use the term 'masterpiece' lightly.
What exactly makes me praise it so much?
The characters, despite there being almost a million of them, are all fleshed out, having their own dreams, backstories, difficulties,
and talents. They all chase their dreams, willing to suffer anything to achieve them.
And that, my friends, is what makes Prince of Tennis so amazing. It's what reignited the fire that once burned in my heart (to be cliche). I loved how they all took their dreams seriously and actually worked and suffered to improve, unlike some other anime protagonists that spouted nonsense about how their freaking love for their teammates miraculously turned them from someone who didn't even know the sport/thing existed, into a prodigy, in the span of months.
In Prince of Tennis, the characters always pushed forwards. They lost at times, facing opponents that seemed insurmountable, but after suffering, they, in a sense, "rose up from the ashes" of their defeat and grew stronger. They made their way up to the grandest stage in the entire world, having jumped through countless hurdles.
One of the phrases I've both loved and hated is "A prodigy at 10, a genius at 15, a common man at 20", which I first encountered while reading 転生者の私に挑んでくる無謀で有望な少女の話 (About the Reckless Girl Who Kept Challenging a Reborn Man Like Me) (and later in Free!). I'm sure, in our childhoods, many of us believed that we were special in some way, that we were different, that we were unique. I, too, believed that I could chase my dreams, but the moment I poked my head out of my small well, I realized the difference that talent coupled with years of hard work made: an insurmountable gap, for me.
After watching Prince of Tennis, I've resolved myself to once again chase my dreams and maybe, one day, be able to stand proud on a stage unlike any others.
That's why I've loved stories about starting over, or about people who never gave up on their childhood dreams, about the strong. And for me, that was what Prince of Tennis was.
A prodigy in the eyes of others, Ryoma Echizen has chased and labored for his dream for years, longer than many, many others. Even when encountering protagonist-like geniuses (i.e. Kintaro, the 'super rookie of the west"), his pride and belief in his skills honed by years of grueling practice allows him to challenge them.
For me, Prince of Tennis held something that no/few other anime or stories possessed. It has been one of the few anime to move me (whether to make me shed tears or to make my heart pound in excitement).
Prince of Tennis is not just a story about kids playing tennis. It's about what it means and what it takes to chase your dreams, and what happens when you achieve them. Neither our protagonist, nor the other characters, are the idiotic happy-go-lucky "somehow I turned into a god after a month despite not knowing that tennis was a thing yesterday" or the "I'm trash so I have other people carry me" types of characters.
Tl;dr: Watch/read Prince of Tennis if you are looking for characters that are always chasing their dreams, and are willing to do and suffer anything to stand on the world's stage.
(Below is just a short summary of each category)
Story - 10
There were quite a few filler arcs, but it was overall fairly engaging and tied together; the results of matches left me wondering several times, too.
Art - 8
The art and animation is fairly mediocre. The characters' faces seem too sharp/narrow, and there are a lot of still frames.
Sound - 10
I don't remember much of the sound, but one of my favorite songs (Wonderful Days) is from Prince of Tennis, so I'll still rate it a 10.
Character - 10
Each character had their own charm, and most were pretty fleshed out and not 1 dimensional.
Enjoyment - 10
Overall - 10
I couldn't stop thinking about it after I finished. (The manga seems to be still ongoing, with a few short chapters released every month. )