It is currently my belief that SWORD GOD performed the most impressive instance of Unretractable Attack of all time, by thrusting a delicate blade one inch deep into a rock-hard granite stone.
Generally, UAs are considered an embarrassment, but SWORD GOD's UA actually evoked a massive cheer from the crowd of 1000+ experts. The result speaks for the feat.
Another notable UA is the one in LOCH where Guo Jing fired a LDA at Huang Yaoshi. Huang Yaoshi turned his body to avoid the attack, but Guo Jing was unable to stop, thus tumbling down the stairs.
Can you think of another UA that could surpass the impressiveness of SWORD GOD's cheer-inducing UA?