i'm going to rank purely based on the toy lines and omit mentions of the tie-in movies and shows. i'll also probably not delve into what i thought of the playsets, as i've never had very many of them.
g1: i always have a hard time deciding between g1 and g3 as my favorite generations, it literally flips every day. i gave g1 the top spot because of the amount of variety. sea ponies, big brothers, sweetheart sisters...every day i'm discovering a new g1 pony i had no idea existed! soft hair, squishy yet durable plastic, the biggest variety of unique symbols across all generations, and they hold up incredibly well considering their age. and they absolutely have the best gimmicks! i love twinkle eyes and perfume puffs. not to mention how cute they are.
g3: my other favorite, and the generation i grew up with. i just adore the poses, the symbols, the colors, the names. g3 pony hair (when it's not a discount pony) is my favorite across all generations, so soft and the perfect length. they're the perfect size for easy displaying, too. definitely my favorite style across all the generations, in terms of that fantasy-horse look that i adore. i literally can't stop collecting g3s, send help! the only thing i'd change is giving us more unicorns. also, more pose variety for baby ponies. why in the world did we never get a baby unicorn? and of course i wish this generation hadn't went out with a whimper with core 7. i actually remember kinda loving core 7 as a kid for a time - i bought that 25th anniversary set of them all and loved how their colors went together. but it overstayed its welcome.
g4: i gave g4 a spot over g2 for purely nostalgic reasons. i'm not a g4 collector, but i did excitedly purchase the very first mane 6 brushables when they first appeared on shelves around 2010/2011. i also grabbed the cutie mark crusaders, cheerilee, and possibly had minty, i can't remember. at the time, it /felt/ like there was a lot of pony variety to come. within the first year or two of production they were producing a few toy-exclusive g4 pony characters along with some supporting characters from the show. it was awesome to see these "background" ponies come to life when i was a kid. of course, the variety sorta stopped right there. i like their chibi look, and i think the pre-movie g4 ponies have this really cute look to them that borrows from the friendship is magic show while not being a literal copy-paste of the show's style directly to toy form. but they're definitely a bit too small and compact. also, what's with the luna toys? princess luna probably has my favorite pony design ever, but in almost all of her toys she's this purple color instead of black and blue. it looks like the only brushable with accurate colors has that post-movie style to it.
g2: i'm waiting for g2 to really "click" with me, but so far it hasn't happened. they're just a bit wonky to me. the lack of pose variety is basically on the same level as g4, but the expressions bother me more. almost every g2 pony seems to have that open-mouthed smile. however, a lot of g2 ponies do have symbols that i really like - golden light comes to mind. it's just a shame that so many of them are european exclusives and pretty expensive to get these days. a lot of them have really pretty color schemes.
g5: g5 isn't very high because of how new they are. i think hasbro is still figuring out what they really want this line to be. i'm loving the little polly pocket inspired sets they're coming out with, and the articulated ponies are lots of fun and have great accessories. but my ideal pony generation is one such as g1 and g3 - a huge variety of characters with soft, brushable hair in a variety of poses. i wouldn't say g5 is that quite yet, but i think it was also unfair of me to get my hopes up so high after g4 was decidedly not my ideal. i do like how leggy they are! i have that large brushable izzy, and her hair is amazing. so thick and nice to comb. her plastic...hmm. it's the kind of plastic where you can accidentally scratch it with your finger nail and leave a permanent mark. (yes, i unfortunately did that.) whereas g1 and g3 are much more durable. the art style of the toys is basically just taken directly from the movie with no changes. if that's what they're going for, can we have some flocked g5s? i love how fuzzy they are in the movie.
g3.5: this covers ponyville ponies as well, right? because i love ponyville, they're so cute. as for the larger g3.5s...i'm just not a fan of the faces at all. they do have nice hair, though. i do wonder how this generation would be perceived if they'd produced more ponies besides core 7.
g4.5: i don't really have any positives for these guys. weird art style and really bad hair.