The most popular choices for favourite generation usually seem to be Gens 2, 4 and 5. Gen 2 obviously had its expansive post-game to make it stand out and was the first sequel to expand on the originals. Even without factoring in the remakes, Gen 4 has a lot of popularity when it comes to things such as its expansions to the lore as well the praise of it having finally perfected the physical/special split and the long wait for Gen 4 remakes certainly garnered more attention to it. Gen 5 was divisive at the time of release, though the majority have come to accept how its changes were for the most part the right decision such as Black/White's plot handling more morally grey themes than the other main series games when it came to Pokemon battling (until the ending acts as a slap in the face to all that) and how not having past Pokemon until the post game provided more incentive in building new teams and experimenting, plus Black/White had the only post-game to come close to HG/SS in terms of expansion and some of B2/W2's features were perfect for veteran fans such as the World Tournament and more difficulty options. Gen 1 also has a lot of favouritism from fans due to being the first, though the amount of "genwunners" seem to have died down as time has gone by due to it being apparent how hypocritical their position is when it comes to bashing later gens' creativity or features.
Gen 8 is by far the most hated generation as none of the prior main games have been lambasted even half as much as Sword and Shield by the vast majority of the fanbase. Gens 6 and 7 probably come next as Gen 6 started some of the problems people had with the franchise fatigue such as a return to the formulaic approach and a lot of hand-holding, with Gen 7 limiting what features people found good in Gen 6 such as Mega Evolutions and the GTS.