I'm going to have to say squats... They're most frustrating for me because I have long legs and it's hard to know how much weight I should put on and when while keeping good form.
(If I use nothing but perfect form for my squats, I can bench the same amount while keeping strict form....it's not cool...)
(If I use nothing but perfect form for my squats, I can bench the same amount while keeping strict form....it's not cool...)