My crown jewel is probably my
Gaea's Cradle. I bought it (and a bunch of other cards) off a friend that was quitting the game 20 years ago for a massive (at the time, since I was just a kid and hadn't even had my first real job yet) price of $20. Best return on investment in my life.
My white whale is
Sliver Legion. I have the other five WUBRG Sliver Legends (2 overlords), but I want all 6, and I just have never seen a great deal on the legion, and I really don't want to spend $40-50 bucks on any MTG Card.