I’m talking about cards that you have in your collection (NOT cards you want) that for whatever reason you don’t use or just rarely ever use.
For me the #1 is Overwhelming Splendor. I just don’t have enough support slots to justify its use in most of my decks, but it’s still a pain to deal with once it hits the board against me.
Dreamstealer is another, because it’s usually used in conjunction with other discarding tools.
Choke is not fun to play against, and would be a wasted slot in most green decks. Guess I play too many blue based decks 
Hixus, Prison Warden was my 2nd mythic and I duped it 2 more times. I’ve never liked the card. It holds the wolves back but it is so fragile and dependent on where it lands. I would rather destroy a threat than slow it down so I rarely play it. But…I have sat powerless multiple times while the Hixus in the corner controls my entire board.
Gaea’s Revenge for me. I see how nasty and game winning it is, especially when you get that one match where the AI manages to cast Mirrorpool before it. But for my part, I tend to like to play cards that compliment each other, and not just Timmy it up with my biggest critters. So I frequently overlook it when deck crafting.
Hixus.
I’m using Cast Out and if Hixus on the field and it was destroyed castouted creatures will no longer be disabled. But using just Hixus is pretty cool.
Convert to frog… i really hate that one. Destroy my creature, and as a joke, leave you with a blue squishy toad! ><
I’ve smashed face with that squishy little toad in my Jace 2 deck with his emblem out… it’s also a fun planeswalker if someone moon prisons your creature since it makes for massive mana gains.
With Jace 2 if you aren’t Mooning your own first creature you are silly!