Let's talk about some underused potions:
Thorns
Behold the worst potion in the game. Turns out that in a game where there can be many enemies at one time on the screen and where the bosses (and even some enemies) have hundreds of times more health than the player, the ability to return 33% of contact damage to one target is completely useless.
It'd be interesting if the potion dealt its damage in an AOE, that way it can get a lot of use against early bosses that use weak minions that would actually die or be greatly hurt by the thorns ping (EoC, Queen Bee and EoW/BoC).
Additionally, I want to ask who the hell thought up this recipe for it
Swiftness
Given how Movement Speed works (or more accurately, doesn't work) in this game I think the move speed buff from this can afford to go to +40% like Tier 3 Well Fed. It will still be somewhat useless once Hermes Boots are obtained (unless I'm underrating that marginal acceleration boost it gives to them) but it'll work very well until then.
Inferno Potion
Behold the second worst potion in the game, with only one real use against Duke Fishron where it pops his Detonating Bubbles (and isn't even good at it, not to mention that the bubbles are his easiest attack)
I think a bigger ring and more knockback would help but other than that idk what to do to this one.
Strange Brew
It restores a humongous amount of Mana as well as some Health but that's not a good thing in a game where Health and Mana Potions are diametrically opposed in use. Pretty much the only time someone would use this potion is when you find the Skeleton Merchant before a Glowing Mushroom biome that grants you access to Healing Pots.
The healing could be raised to 90/100 but really it just needs a full rework since the core concept doesn't work in the first place.
Titan Potion
Ok I don't want to say that Knockback is a worthless stat but this potion kinda is? I think the real issue is that it's locked to Post-Skeletron with the random Bone in the recipe, so it can't be obtained early on where the effect would matter the most.
I also think it'd be nice if it increased knockback by a flat 1 on top of the 50% increase, that way weapons with 0 base knockback can benefit from it.
Flask of Nanites
I guess it can be useful in the Dungeon but that's about it. Nanites just need a new effect that isn't as tricky to work with as Confused, to be honest.
Flask of Fire/Poison
These two flasks should be somehow made obtainable earlier on, because Post Queen Bee is right where their DoT becomes worthless.