2003 MW GP takes the cake imo. It was the one with the most number of memorable fights.
Chuck, Rampage, Wanderlei, Overeem, Sakuraba, Bustamante... so many classics.
Wanderlei vs. Yoshida was an absolute war
Liddell vs. Rampage was insane.
Rampage vs. Busta was insane
Liddell vs. Overeem was crazy too.
Rampage vs. Wandy final was shocking.
I thought the MW from 05 was a bit of a letdown tbh, considering it was the most stacked names-wise: the final between Shogun and Arona was a one sided slaughter, Arona vs. Wanderlei was quite boring, Shogun vs. Rampage wasn't competitive either... Rogerio vs. Shogun was the best fight of the tourney. Kondo vs. Henderson is underrated. Two upsets by Overeem with Igor and Vitor, which was cool. Arona's destruction of Sakuraba was infamous. Wandy vs. Nakamura was not bad, though the rematch with Yoshida wasn't as good as the first fight. Still super close. Arona with Lister was a great ground scrap.
The HW 04 GP was very good, if we take the final rematch between Nog and Fedor III as the culmination of an insane trilogy, it was the best final of any of the GPs and one of my favorite fights of all time. Not to mention Sergei vs. Nog which was a war, the insanity that was Fedor vs. Randleman (same night as Rampage's slam of Arona...), Randleman's insane upset of Mirko... Heath vs. Nog rematch which ended with the famous anaconda, Kharitonov's destruction of Ninja and Schilt as breakthrough performances, Fedor's obliteration of Ogawa...
The OG 2000 GP had extraordinary moments, though it was messy and inconsistent: Saku vs. Royce is one of the most important fights in the sport's history. He then proceeds to fight Igor in a competitive bout. Coleman's destruction of Igor was also impressive, even if one sided for a final and thus a bit underwhelming. Kerr was hyped but already was a mess with painkillers when Fujita stopped him, which was a shame. Fujita threw the towel with Coleman and just cashed in, which was a bummer...Mezger vs. Saku was fishy, to say the least... overall, a bit shady, but also historic tournament.