Well, I like (more or ...less) all generations. The list goes from best to worst:
G1 - I was too young too see them in stores, but my childhood friend had a Sweetheart Sister. I remember stumbling upon Dream Valley for the first time and falling in love with all the old ponies! I don't have many of them, because they're rather HTF and expensive where I live, but I truly do love them! The ponies were perfect and creative - not to mention the gorgeous artwork!
G2 - I grew up with them, of course I love them! They're so slim and fragile, they remind me of The Last Unicorn
I love their ethereal looks. I think that the gem stone eyes were really good idea.
You can imagine how strange G3s seemed to me when they hit the shelves....
The only thing that I dislike is the lack of interesting poses. The G2 ponies don't seem so interesting to collect, because the poses are too similar.
G3 - I like how they're a return to roots! Finding a G3 pony in a thrift store got me into collecting
Lots of interesting poses and characters!(Of course pre-Core7)
G4 - I remember when first photos of the new ponies were shown - half the people on arena found them ugly. Everything has changed after the first episode of the show has aired. The same thing has happened to me. I think that they're sort of cute, but not interesting enough for me to start collecting. I bought one Rainbow Dash and one Fluttershy way before the brony business started and that's it - I don't really see the need to have more. I dislike how tiny they're
And they don't look like horses at all...
G3.5 - the messed-up prototypes of G4, the missing link of pony evolution
Well, the Newborn cuties were sort of cute, but the rest was just like G4, except all lose and no gain...
I also really love (good) fakies, I've even started a blog on them
I know that most collector dislike them, but to me they're like the undiscovered story of MLP!