I’m looking into getting some new string. I really like the stiffness of yysl type x so I was looking for a string that was a little bit stiffer for whips and slacks purposes.
I’ve tried YYE string, Kitty string, and Blueprint string. Blueprint is the stiffest by far.
Chaos 422 are pretty stiff.
Twisted Stringz Mojo are nice and stiff.
Original Epic Strings, can’t remember what the model became known as.
I forgot to add that it was Kitty string Standard that I’ve used before.
Barbed wire
i have twisted strings grey matter (might still be in stock here), and they are literally the whippiest string i have ever used :o :o :o
There is zero stiffness to gray matter. Super soft, thick, and plush. Fast whips due to density of material, yes, but fast whips != good usable whips
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ooooh ok, lol got my info mixed up 
actually, now that i think about it, the “Atomic Wonder” colorway of the original Toxic Strings (or just the original in general) were pretty stiff imo, although i don’t think they still make it
Gstring NOLY is super stiff, but I believe out of production. As mentioned above, the original Toxics were very stiff, but the newer iteration (Base), while still on the stiff side, is somewhat softer.
Toxic Dragons are pretty darn stiff. Crazy fast whips. Petty rough at first, but after a few hours of play they break in nicely and still are my best whipping strings.
For comparison, the other strings I’ve used in terms of whipping ability; worst to best:
YYF poly
YYE Expert poly
Kitty Fat
Blueprint
Type X
Kitty XL
Slackline Poly
Dragons are by far my favorite, Kitty XL is my second favorite.
Twisted Stringz Type A for sure. They feel like plastic untill they break in though. I’ve been using one for about three months now in my Chik and it feels better than ever. Also I’m not if they are still attainable anywhere but theres theses strings called Angel hair that are the longest lasting and whippiest strings I’ve ever played.
I would have to agree with GregP in that Chaos 422 is the stiffest I have used as well. I managed to stock up on them before they went away, and they are so stiff they can actually cut my TH index finger when I throw aggressively.
Personally I think the BEST string for stiffness without being so hard on your hand is the Toxic BG1.
I believe they are transitioning the BG1’s out though, and replaced them with the Toxic “Blends”
I HIGHLY recommend them, and they are available on their web-site as well as YYE.
Agree 100% on Toxic Dragons. Probably, by far, the stiffest string I’ve ever played. They break in great and last FOREVER!
Kevlar hands down
I agree, they were way too stiff and rough for my liking but if you’re into that kind of thing then I’m sure you’ll love them. Definitely the stiffest string that I’ve personally tried. 
do they still sell them i remember them… some of the best string out there
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Blueprints are the stiffest I’ve used. I’ve only used highlights, YYE poly, and kitty. But blueprints are a solid string, might switch to them cause they’re cheaper than kitty.
They just put up some more for sale, but they sell out real fast.
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Definitely NOLY. You could use them as walking sticks.
Now the hardest would unquestionably be kevlars. Those things were like piano wire.
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Dragons are a double edged sword for me. I absolutely hate putting a new one on, because I agree, they are way too stiff and rough at first.
But once they’re broken in, they’re like my perfect string. I wouldn’t change a single thing about the performance after break-in. And it just amazes me how long they last in that condition. So they’re totally worth the effort in my opinion; no pain, no gain!