I suck at naming only one favourite, but the ones that have impressed me most are Star Trek (original + TNG), Babylon 5, Firefly. I'd name Futurama as one as well, it's the most brilliantly written comedy series I've seen.
Farscape and Eureka are other personal favourites, although I understand why they are relatively less hyped. Not sure if Millennium counts as sci-fi, but it is another personal favourite. X-Files was also great for a long time.
Of new ones I've liked Mandalorian S1.
I have Expanse on my watch list next, have high hopes for it.