This is a difficult question for me because my personal favorites are not necessarily the seasons that I feel are the "best" seasons.
Personally, SuperS and Sailor Stars are tied at #1 for me in terms of favorites, with S in 2nd, and R and Classic tied for 3rd. I have a lot of nostalgia for Sailor Stars, and I love the general appeal of SuperS overall. The show feels more mature in Sailor Stars and I really enjoy characters like Kakyuu, the Starlights, Galaxia, the Anima-Mates, and Chibi Chibi. The whole thing feels cool and mythical to me. SuperS, meanwhile, has some of the best filler episodes in Sailor Moon contrasted with entertaining villains and a general creepiness that I enjoy.
In terms of which season is actually the best, though, I do think that S is the "best" because it has the most consistent story, engaging villains, a great cast, the Outers, a complete plot...basically, it's a stronger season than both SuperS and Sailor Stars, but I can't say I love it more than the latter two seasons.
I think Classic is the worst season, with R being only slightly better. I just feel like not much happens in either show. Sailor Moon's initial uniform is my least favorite of her looks (I much prefer Eternal Sailor Moon followed by Super Sailor Moon) and the plot of Classic kinda bores me, though at the same time it has some of my favorite episodes in the entire series, I know that's a weird dichotomy but it's true. R has the break-up arc which I can't stand, along with some pretty bland villains IMO and Chibi Usa at her worst (she's much better in S and SuperS). The Makaiju arc at the beginning doesn't do anything for me personally. But at least the Black Moon arc in R is more interesting than anything Classic was doing. Mamoru getting brainwashed was a huge bore for me and I don't like the Rainbow Crystal arc. In general, the first arc of Sailor Moon is always my least favorite in almost every single adaptation, with the exception of PGSM because I feel like it was actually done well there (and it was the only arc adapted...lol).
So basically, for me personally: S is the best but SuperS and Sailor Stars are my real favorites despite being not as good; Classic is the worst and R isn't much better. But I love them all so being at the bottom isn't exactly a bad thing.