For me, I go with a top 300 ranked Daniel Nestor defeating then no.1 Stefan Edberg in 1992 I think in Davis Cup.
Both awesome runs (especially with the 'Vladiator' story - he watched Gladiator numerous times which inspired him to makes the SFs as a lowly-ranked qualifier, heroic stuff), and Dewulf reached the QF the following year as well, in epic Popp/Pironkova-esque fashion.Wimbledon 2000 R64 : Voltchkov (237) def Pioline (6) 6-3, 6-3, 2-6, 3-6, 6-4
The gap is less impressive but he made it in GS and he also managed to beat El Aynaoui (16) during this tournament to reach the SF ranked 237 !!
Thinking about Voltchkov made me think about Dewulff also and his run as qualifier at RG 1997 to SF, where he beated Corretja who was in the top 10 at the time, Norman and Meligeni.
karlovic vs number 1 and defending champion L. Hewitt in Wimbledon 2003.
Karlovic was 203rd, Hewitt was 2nd and defending champion2003 Wimbledon: Defending Champion Lleyton Hewitt being ousted by Qualifier Ivo Karlovic in the 1st Round
Yes and he ditched his planned doubles partner for the Australian Open juniors that year after the title: Roger Federer.Young Lleyton Hewitt (16 y 10 m) won the singles title in Adelaide in 1998 to become the lowest-ranked (#550) winner in the history of the ATP tour. He beat Andre Agassi in the semis. Following the win his ranking climbed to #200 in the world.