I'd say Lavender. It has zero effects that I use/need, and the potions it can be used for aren't very valuable. I only ever use it until I discover all of its effects, and then never touch it again.
As for mushrooms, Mora Tapinella is awesome. Lingering Damage Health is a fairly common effect, and it's also insanely useful. Plus those potions sell for quite a bit.
Lavender is pretty bad. I tend to gather it early in the game when I'm struggling for money but then after a little while it isn't really worth 0.1 carry weight to me
On my current game (sneaky archer) I've been trying out poisons a lot more and lingering damage health is great! The ingredients aren't that common but there's just enough around that I've never ran out, and eventually when I got the perk green thumb, a garden and greenhouse it was pretty easy
Daedra Heart... it's a much better smithing component.
Giant Lichen, btw, if you are in to poisons, can be combined with Skeever Tail (common) and Deathbell (extremely common) to create a really nasty poison that I use often. Damage health/ravage health and (key) weakness to poison.
I have no idea what daedra heart does as an ingredient because there's no chance I'm using such a rare ingredient for alchemy!
Thanks for the tip, I'll try that poison out tomorrow. I imagine that's going to do a big chunk of damage? I'll find out!
On the topic of giant lichen it must be the most difficult ingredient to spot. Grass pod and jazbay can be hard to notice too, but giant lichen is a nightmare