Not a tough call at all: Olessia Dudnik from the 1989 Worlds had one of the most difficult routines I have ever watched!
Kui and Shawn are really close and I believe the most difficult routine by Catalina Ponor was at the 2006 Euros. She did one of the most insane combinations ever: Onodi-aerial walkover-Kotchetkova-BHS-layout to two!
Gutsu was not the first to attempt a standing full – as far as we all know Priakhina was. The first to do a full on beam out of a TO was Shishova.
I vote for routines in the context of their performance:
Memmel 2003 (although I don’t like the routine as a whole much there is stil a lot of hard stuff in there – I actually liked Memmel back then because of her guts and she did have some originality – not so much anymore)
Kui YuanYuan is a goddes on beam. She was totally robbed in 1997. Tie break rule WTF!!! Anyway, I have to agree with some other comments. Dudnik’s routine was way ahead of its time. Groshkova, difficult but sloppy but still ridiculous. Gutsu…nuff said! Don’t forget Zelepukina who basically did Dudnik’s routine but instead of a triple twist dismount, she did a RO-BHS-double back. Finally, Laschenova’s routine was very nice as well. Huge layout 2ft and first woman to do BHS-BHS full in dismount.