I think Wimbledon was the best, because of the best final (5 setters between 2 of the big3, number 1 and 2 seeds, almost 100 winners from Federer, end at the 5th set tie break, drama etc...) and also other quality matches (Nadal/Kyrgios, Djokovic/Hurkacz, Goffin/Medvedev, Nadal/Federer).
Then I would say Roland Garros, because of great matches all tournament long (Mahut/Cecchinato, Hoang/Verdasco, Tsitsipas/Wawrinka, Federer/Wawrinka, Djokovic/Thiem, Nadal/Thiem)
Then the US Open just thanks to its final. And in last position the AO with only Tsitsipas/Federer to remember (and zoning Djokovic in the final)
Fun fact, i genuinely couldn't remember who played in the Australian open final this year immediately and that says a lot about how memorable that was lol but it's obviously Wimbledon, that final is still fresh in my memory and the 40-15 meme won't die anytime soon. Also Halep going GOAT mode in the WTA final was also incredible to watch. There was a few other great matchs there and for me and other Murray fans it was nice to see him in the doubles with Serena. Best slam this year by far.
I think Australia was a big step down on usual this year, which is a shame because it's probably been the highlight the last couple of times around. I'd have that last.
US Open would be 4th, it had a few good matches, but only 2 really great ones from what I saw.
The other two would be close for top spot, and that also makes a change from recent years, because they've not necessarily been the best recently. Wimbledon had the better final, but the first two sets of the Roland Garros one were probably better than any but the fifth. I think Stefanos-Stan, Khachanov-Delpo, and Mahut-Cecchinato were the 3 best of RG and probably outpace Medvedev-Goffin, Schwartzman-Berrettini and the final for me, so...
4. Australian Open
3. US Open
1. Roland Garros