When the dust settles, I think the general consensus for the best seasons of Discovery will be Seasons 1 and 5. People who like what the show started off as the most will prefer Season 1. People who like what the show ultimately became the most will prefer Season 5.
The fifth season isn't over yet, but I've seen enough now to figure this is how it's probably going to go.
Season 2 had the switch in showrunners part-way through and it's really obvious. People who say Season 2 is their favorite tend to be fans of SNW and like it as their favorite because of Pike. The most common complaints specific to Season 4 are that it dragged on, became "too emo" (neither or which are my opinion of S4, but the ones I've noticed the most), and that all the rough edges were sanded off (this one I do agree with). Season 3 crammed too much in and people will either love the cause of The Burn or hate it, and they'll either like The Burn in and of itself or hate it.
Interestingly, I think season 2 is the second worst in the franchise, behind only PICARD season 2. And I absolutely love SNW.
Season 1 was just a disaster, but it did at least bring STAR TREK back to tv, so I give it credit there. (Pretty much the same feelings I have for STAR TREK '09.)
I agree that season 4 dragged too much, but at least it was the first (and so far, only season) to stick the landing. Season 3 flowed a bit better overall than season 4, but the Burn's cause really destroyed the ending.
So far, season 5 is better than either 3 or 4 at the moment... it's feeling like a slight slump, but it is nowhere near as bad as season 4's slump.
I have a feeling you are right... that season 5 will be looked at as the best season. (Still have 5 more episodes to go, so I hope they don't find a way to screw it up.)