For me it was manga more than anime because I like reading more than I like watching and because I already really, really, REALLY liked comic books.
I can't emphasize this point enough : I've basically been reading comic books my entire life as some of my earliest memories were learning to read The Smurf and Lucky Luke with my older brother, and a for-children comic adaptation of the Bible with my mother because that's what happens when you're in a modern-but-not-too-modern catholic-but-not-too-catholic family.
So picture this : you're 14 or so and you are very much into comic books, all your life they've been your favourite thing BUT old-school upbringing obliges, you've only really been exposed to Clear Line, Atom Style and Paris-School franco-belgian books. And you're off to spend the summer with your boy scout group at camp, and everyone brings some books because people will need to pass some time. You bring a couple sci-fi novels. One of your buddies brings in the first two volumes of Air Gear.
You were NOT ready for this.

It's weird, it's violent, the art is dynamic, there's a ton happening all at once, the action is divided like nothing you had seen before.

By the time camp is over, you've read and re-read the two volumes more than a few times (as well as one single volume of Hunter X Hunter during the Greed Island arc but that was confusing more than anything, as you can probably imagine) and your buddy lets you in on a few other names you can check out when you're back home and figure out how to find scan uploads.
One of the names you manage to remember because it's short is Soul Eater which was barely coming out at the time, it must have been in 2007 or so, the translation was just hitting the bookshelves with an almost synchronized release of the anime since the local publishers had lagged a bit.
You were EVEN LESS READY. At least with early Air Gear you can kinda get the premise of "rollerblades on steroids + street gangs". But THIS is something else.
It is much weirder.

More violent.
The choreography is nuts.

The visual dynamics are nothing you've ever seen.
It was so hard waiting for new scans to be translated, I taught myself a lot of english so I could read the scans before the local translators did they job a bit later.
And THEN
You learn that there's an animated version.
FOR BOTH.
While both were quite neat (until Soul Eater had to deviate after catching up with the source material and really shat the bed with its ending), they didn't have the horizon-expanding effect the manga had.
Although I would very much be lying if I said the Air Gear soundtrack didn't play a very important role in breaking my "metalhead teenager" stereotypes against electronic music.
I've re-read Soul Eater recently and I must say, as flawed as it is (and it really is, especially when you get further and the pacing gets rather wonky), this type of hyperstylized, weirdness-over-substance method in both visuals and writing is something that feels distant now that a lot of the stories we get in manga are to some extent meta or try to have "a twist" or commentary or play on stereotypes. There's something really genuine in how weird everything is just because it's cool to have weird stuff happening.
At least it's refreshing when every other new upload you see seems to be completely explained by its title and contains the word "reincarnated".
I dunno man, there's something about 2000s wacky style-over-substance weirdness that makes it a very good cure against cynicism.