Oh, I forgot to add that duckmite can power a rocket, and o of s can kill (or at least stun) a giant, cranky king sturgeon.
http://www.classicmarvel.com/base/adamantium.htm
True. I was confusing that aspect for Vibranium which is in Wakanda and the Savage Land(which I think was destroyed by Zeladane).
A Universal solvent is handy to get out those nasty stains…along with the carpet…and the floor…and the basement…and walls…the neighbors… It’s a bugger to package, though.
Weapon Salve, from medieval alchemy, mentioned in Umberto Eco’s The Island of the Day Before. If applied to a blade (and maybe the person wounded by the blade), then whatever is done to the blade will also be felt by its victim.
Datura, from Ryu Murakami’s Coin Locker Babies. An experimental drug created by the US Army that causes the subject to go into a homicidal beserk rage that doesn’t end until s/he is dead. All stocks were buried deep under a reef in the south Pacific.
Forgot one.
Hard Vacuum. Invisible, weightless, completely undetectable (there’s literally nothing there), but objects made from it are completely functional and nearly indestructible.
Though the government denies any knowledge of research into hard vacuum, recently unearthed documents mention the contents of a crate somewhere in Warehouse 23…
How could I possibly have forgotten the substance that made X-com UFO defense possible: Elerium!
Probably Dune’s spice, because it was so powerful and rare that it sparked a war between two huge, rival houses.
Then again, Wolverine is really cool, so Adamantium is a close second.
Where’s the barkeeper? I am in need of a Pangalactic Gargle Blaster right now. And there you have my favourite fictional chemical 
Cheers!
Cavorite The gravity neutralizing gas form H.G. Wells, The First Man In The Moon.
Having had a chance to look up the Skylark series, I notice they later abandoned arenak as a weak and wimpy building material, and went over to inoson, which was much stronger, and also purple.
If we’re talking fictional drugs … “Doc” Smith also gave us bentlam and nitrolabe … longevity in the Instrumentality of Mankind was assured by regular doses of stroon (the santaclara virus, extracted from the giant mutant sheep of Norstrilia) … the king of fictional drugs, though, has to be Philip Dick, what with Can-D, Chew-Z, Substance D, and, of course, Ubik.
Then there’s Jack Vance … I can’t remember all the poisons of Sarkovy, but I do recall that cluthe was pretty nasty.
For a fictional building material, nothing can match “ice.” In David Wingrove’s “Chung Kuo” novels, the future earth, ruled by the Chinese, consists mainly of seven continent-sized “cities.” Each city is a single structure, a 100-story arcology made of “ice,” a super-strong, lightweight plastic.
Fayalin, anyone? Never could tell what that did to you - it was described as “stimulating, but non-intoxicating”. Strong coffee, maybe?
Another drug was hadive, which you smoked, and made you smile idiotically.
Bentlam was the stuff you chewed after getting drunk. Even as a child, I knew this kind of mixing drugs couldn’t be good. And I was rather surprised when Kim Kinnison shrugged off his addiction to the stuff as “nothing I can’t handle”.
Dagal was another super-metal, mentioned only in passing, but “considerably more resistant” than arenak. Nothing like inoson, which was the theoretical limit.
I liked lux and relux from Campbell’s The Black Star Passes, which were solidified light. He used relux to make his heat rays in Islands of Space, one of the first sci-fi novels I ever read.
Regards,
Shodan
Nobody has mentioned transparent aluminum? Very handy if you plan on taking any humpback whales back with you into the future.
Hey, come to think of it, why did the tank have to be tansparent? Why couldn’t they have used some strong opaque material? It’s not like the whales needed to look outside during the trip.
They didn’t need it to be super-strong. They just used Plexiglas.
The OP specified fictional chemicals. Not ones that are real but the govt. won’t reveal to the public. 
Yeah, but they wanted to use transparent aluminum, only it hadn’t been invented yet. They had to settle for industrial-thickness Plexiglass. My point is, they could have just used regular aluminum or some similar strong-but-lightweight material. I don’t know of any reason it would have to be see-through for such a short trip.
Yeah, arenak was “only” 500 times stronger than steel, while inoson was 2000 times stronger than steel.
But try making window panes out of inoson! I mean, sure, it’s transparent, but it’s purple! And that’s just not right. (Unless you’re making sunglasses or a tinted sunroof.) Arenak is transparent and clear, like a see-through indestructable substance ought to be.
It wouldn’t even have to be lightweight. Compared to the 400 tons of water they were carrying, another few tons wouldn’t much matter.
Would I be dating myself terribly if I mentioned (since most of the other neat fictional chems have been hit already) that nifty stuff, Adhesive-X?
[sub]Although, I bet the Baron hadda gulp down a buncha Mentats to think that stuff up in the first place, and even more Buffout to set up that giant vat he was brewing it in.[/sub]
Oh, I forgot: transparent aluminum.