Instant Damage, Poison and Regeneration (Other than the already existent Golden Apples. What could also work would be to include ingredients to make potions in the map all over.
Yeah, exactly.
Instant damage 1 = 4 hit kills on anyone. Literally anyone. Armor doesn't effect instant harming unless it's enchanted, which still effects it only very slightly. I know from experience. So long as someone has 4 instant damage potions, then they pretty much win the fight before the other guy can see them coming...the fights don't feel fair. (Instant damage II potions 2 hit kill people with direct hits...they'd be dead in a sneak attack no matter what.)
Poison slows down the target far too much for far too long, and also puts them down to half a heart. Throw a poison potion on someone and they are as good as dead. Someone could literally come up and punch them once and they'd die. They are also slowed down by the constant damage, so...it's over powered.
Regen makes you pretty much win a fight if you take it at the right time. You'd be surprised how powerful it is. In an even fight against someone who is about just as good as me in PvP, about an equal number of hits, etc, I still live with 6 hearts and counting. (With regen II) Even regeneration I would still leave you with about 3 hearts, and that's a huge advantage. This isn't even mentioning running away to regen, coming back at the opponent, running to regen again... eventually, it will be them with 10 hearts and the opponent with 1 and a half because of regen. They are way too powerful.
My point I'm trying to make is that these potions are so over powered, that if they were added, it would be essential to have them to fight someone. That means you would be forced to get lucky enough to get one of these potions in a chest, or you pretty much lose no matter what. It makes it too much of a luck game and less of a skill game. (I know, chest routes, blah blah blah - no. I don't care how good your chest route is. I can and have beat people who had full iron + a diamond sword when I only had half chain and a stone sword. I was perfectly capable of getting full iron on that route, I usually did, but luck is a factor, and I want my skill to be able to make up for my luck. Adding potions just makes it so they can be even more overpowered, and I'll have an even harder time relying on my skill and more of a time relying on my luck.)