Ranking all gens from best to worst:
>Gamers
FBK, Korone and Okayu are rightfully popular and successful, Mio is less popular but is still really likable and has a loyal fanbase. Cover struck gold with this gen.
>4th gen
A strong lineup with no weak links just like Gamers but none of the members are as notorious as Fubuki and Korone except maybe Coco. All members have their own niche with fans that are really into that shit e.g. Luna with her piano, Watame with Night Fever/Watame no Uta and Towa with Apex.
>3rd gen
Hit or miss. Pekora, Marine and the slut that is Rushia are the big hitters, Flare is the one with a cult following and Noel is shit.
>5th gen
Probably the most consistent gen, all members are pretty good but no one has the same notoriety as Pekora and the like. Their collabs are pretty fun.
>0th gen
Consistently good like gen 5 but while every gen 5 member is someone's oshi good luck finding someone who has Sora or Roboco as their oshi.
>2nd gen
Hit or miss like gen 3. Aqua, Shion and Subaru are amazing and deserve their success, slut Ayame is less popular but has a cult following, Choco is shit.
>1st gen
Mel and Aki ruin an otherwise good gen. FBK, Haato and Matsuri are great, though.
>HoloID
A good gen, it just happens that everyone in it is that sort of vtuber you only watch when they are in a collab with someone you like or if there is no one else streaming right now. You never see them listed as someone's oshi.
>Holostars
Wait, this was a thing? I forgot.
>HoloCN
RIP
>HoloEN
Popular in the same way that Pewdiepie is popular: Gura, Kiara and Amelia appeal to the lowest common denominator with shit like dumb memes and twitch talk and Calli appeals to ironic weebs with cringy raps about "moe that sells". Ina carries this gen with the only watchable content in it: her drawing streams.