Going off of my current favorites, and limiting myself to one performance per actor
Male:
1. Robert DeNiro - Raging Bull (To me, the gold standard of method acting, and never becomes boring, nor feels like someone's just showing off their physical transformation)
2. Daniel Day Lewis - There Will Be Blood (Was trying to limit myself to just one per actor, and it was tough to narrow it to just one DDL performance, but I come to this one. Next week it could be a different one.)
3. Jack Nicholson - One Flew Over The Cuckoo's Nest (the iconic Jack performance for me, never gets old, plays to his strengths, and even though I shouldn't, he makes me love the rascal)
4. Donald Pleasance - Halloween (It's hard to picture it without him, and often overlooked because it's not really showy, but the way he monologues about pure evil, even in the lesser sequels, he's just magnetic. Plus, it was his idea to have an "I knew it" expression at the end of the film.)
5. Heath Ledger - The Dark Knight (totally forget that it's even Heath in the role, still riveting to watch on screen after 10 years. Just watching his eyes dart around, commanding a room that doesn't want to take him serious, but does anyway...it's just perfect.)
Female:
1. Jodie Foster - The Silence of the Lambs (I'm not sure why i go to this as my #1 actress performance, but I always do. Maybe it's because she seems so effortless in the role, it deceptively easy. She's excellent, and iconic.)
2. Ingrid Bergman - Notorious (I pretty much love Ingrid in anything, kinda like DDL, but Notorious is my favorite movie she's in, so I'm probably bias towards it. I feel like I fall in love with her every time in this one.)
3. Hillary Swank - Boys Don't Cry (blew me away how well she nailed a cocky southern boy, and like Jodie, made it look effortless. I had seen her on 90210, and The Next Karate Kid, and didn't care, but I became a fan of hers with this.)
4. Michelle Pheiffer - Batman Returns (Underrated I think. She's superb, and the amount of desperation, sadness, rage, and love all rolled into those watery eyes at the end, it's rare I find it convincing someone can pull all those off at the same time. Plus, she does just the right amount of camp to keep it fun.)
5. Viola Davis - Fences (goes toe to toe to what I think is also Denzil's best performance, and chews him up and spits him out.)