G3. I got into MLP with FiM/G4, but once I saw the G3 toys, I started becoming a collector. They're just so beautiful, there are so many really pretty colorings and themes/symbols.
I have Autumn Skye out on my desk, hoping to summon autumn!
For me it was G1 that got me into ponies (childhood collection) but G3's low prices (at the time), diverse poses and great quality converted me to G3.
Autumn Skye is one of my favorites!
I have ponies from all gens in my collection with the exception of 3.5 because in my opinion the toys looked kind of like ducks.
I also don't care for the G4 reboot. I prefer the original G4 head mould and screening. Also, the very first few Playful Ponies still had neat designs. I viewed them as mini versions of G3s (Rainbow Flash, Bumblesweet and Skywishes all made it into early G4).