I have been following Le Tour since 1975, the Giro since 1982 or 1983, and the Vuelta with Hinault and then consistently since it changed dates in 1995 (?). Obviously, these are only my preferences.
For the 21st Century: BEST
Giro: obviously, the '10 edition had all the ingredients: deep field, great route, suspense, big stages and big battles. '05 had suspense, lead changes and the Sestriere stage was epic. Then I loved the battle for the podium in '17, thanks to the poop-gate the gaps were small until the end, for a Pinot fanboy, a great GT. The same goes for the '18 edition, with its wtf last week. My honorable mention goes to the '07, with a loaded (hehehehe) cast and Andy Schleck's breakthrough on Zoncolan behind the two Saunier Duval.
Tour: too often dominated by one team, in terms of entertainment, my frenchness hates to admit it but it's a fact: Le Tour often sucks. And when it doesn't, there always seems to be a "what if". '19 still hurts, but what a Tour and a wtf conclusion. '14, "what if Froome and Contador had remained in the race?" To me, in Sheffield and on the cobbles, Nibali proved that he was the best, regardless. '03 was the only Lance with suspense, avoiding the Beloki crash, Ulle falling in the rain...maybe '10 was the best after all, but again, the chain-gate. OK, '19 it is.
Vuelta: '06 was an intense one with a thriller of a third week between Valverde and Vino. I'd put it at the top of my list, tied with the '12 edition. The '16 Quintana is a distant third.. Honorable mention for '15, the Dumoulin revelation and heartbreak. Simon's revenge wasn't bad either.
WORST: '12 Tour, '12 and '19 Giro (favorites riding defensive and an opportunistic rider wins). '16 Tour in the next tier due to changing rules in the Chalet Reynard stage, thus making the Froome run a run that never was. '17, meh minus like the '11 Vuelta.