From Worst to Best (just to end on a positive note):
G2 The Friendship Gardens Computer Game?: This is the only part of the franchise that I have NO connection to or love for. I have no real feelings for this entire generation. I only know that the toys have more real-life horse-like proportions. I don't know any of the characters. I don't care about any of them. I wish someone would do something with these guys just so I could form an opinion on them.
G3.7 Newborn Cuties: I mean...come on. The best you can say about this is that it was mercifully sort lived. The animation was bad, the characters were all babies, it was just not a good time.
G4.4 Hello Pinkie Pie: The characters sound wrong. The characters often do gross things. And the whole thing felt rather pointless.
G3.5 The Twinkle Wish Adventure and Others: This is basically what everyone THOUGHT G3 MLP was. Girly, shallow, brightly colored and little else. It still has moments of sweetness to it, but it's a lesser version of G3 proper.
G1.5 Tales: I don't hate it, but it is just a bunch of teenagers being terrible to each other most of the time. It has some memorable characters, though. The inclusion of UFOs and the first Alicorn is also something it contributed to MLP mythos.
G4.5 Pony Life: This was sometimes painful to watch. Seeing the characters we all know and love turned into Teen Titans Go characters with gross out humor and internet memes. But, it did have some ambition. They had multi-episode arcs and Season 2 has a season-long storyarc. Some new characters were introduced...though none of them were very memorable. They tried....I think they failed, but it WAS a stepping stone that lead to Tell Your Tale, which is legitimately good.
G4.3 The Stop Motion Shorts: These are just cute. Not much to them, but I liked them.
G3 Minty, Minty, Minty: This is surprisingly fun to watch. There's occasionally some good lines here and there. And it's kind of the prototype for G5, since it's storyline is all about reconnecting a bunch of tribes of ponies who got separated. Shame we never found out why. Minty's love of socks will be permanently burned into my brain. And I'm okay with that.
G1 Ponies and Friends: There's just something about 80s cartoons, you know? I'd actually put this on par with other 80s fantasy cartoons like Dungeons and Dragons, myself. It's darker than you might expect, at times. Emphasis on "at times", though. Because, it does have it's moments, but a good portion of the cartoon is filler, too. This is really the only generation with "real" humans, which makes it stand out, even though I'm not sure if Megan's two siblings were necessary. Some characters are really memorable, but it has such a rotating cast that you never know who you're going to get episode-to-episode. It's uneven, as a lot of 80s cartoons are.
G4.2 Equestria Girls: This series of movies and specials is...uneven, I think. It starts pretty strong, gets better, then kind of fluctuates for a while before just kind of stopping. I kind of wish we got some sort of conclusion to the story, somehow. As it stands...this is G4, but with characters who didn't really grow or change the same way that main G4 did. We did get Sunset, though. And that's worth a lot. It's fun...but, mainly as a spinoff of G4 rather than it's own thing.
G5 The New Generation: This is such a hard generation to place, right now. The original movie is probably the best Pony movie, easily. And we're actually getting a LOT of media. But, it also feels like this generation is having a slow start. The decision to make Make Your Mark a 3D cartoon on Netflix feels like it's wrapped a anchor around the neck of this franchise, even if I am enjoying most of it. Make Your Mark gave us Misty, who I'm liking so far. I think the comics have been okay, even if they are drawing out the Discord storyarc too long for my tastes. And Tell Your Tale, the free Youtube series, might just be the best of the bunch. TYT has some really great humor and characterization in 5-minute bites. I've grown to really like the characters. But, it's got a long way to go to dethrone the champion.
G4 Friendship is Magic: Yeah, this isn't even a fair fight. This is the generation that spawned a fandom. A generation that changed how people saw "girl's" cartoons. It's had FAR more impact than any other generation of My Little Pony. It's Hasbro's biggest success out of the Hub experiment. It got over 100 comic issues (probably closer to 200 if you include spinoffs). The characters are still showing up today. It's still getting fanart, maybe even moreso than the current generation. Did it go on for too long? Maybe. But, I'm still missing it more than any of the previous generations. I like most of the other generations, but I love this one. There really was a bit of magic in this show, and I don't think that will be replicated anytime soon.