I started out by collecting only the ones I wanted as a child, adding to my original childhood herd, but then it grew and then G3 came out... now I collect all 4 gens. I love making lists and ticking them off so I collect all types of ponies and playsets and accessories. All other collections have long gone to make way for more ponies.
G1 criteria: loose and in minty condition where possible, complete with accessories or I will spend ages hunting down any missing parts
I collect backcards and playset boxes but try not to buy MIB as I will only open them and feel guilty about it! I collect all USA and UK including variants. I also collect Nirvanas - rainbows, Blossom variants, collector pose variants or just any that I think are pretty. I collect vintage merch, comics, softies, porcelains, petites and mummy charms. I have one Dream Beauty but no doubt she will gain a companion at some point. I am only missing 3 USA, 4 UK/Euro and 14 MO to have a complete set of brushable G1 ponies.
G2 criteria: MOC/MIB and I don't open these! I do have a few loose, some are hard to find so will be super difficult to get MOC. Still got 47 MOC and 1 beanie to get to complete my G2 collection.
G3 criteria: All were purchased new and most are now loose, but I have kept all of the packaging (yes my attic is full of boxes!) - some special ones I even have doubles so I can open one and keep one mint. I have every variation of G3 ponies, except the giant blank pony and open-mouthed Ribbon & Hearts, but not every packaging/accessories variation. I have a huge army of Pinkie Pie to show all the variations of packaging and merch. I have approx. half the G3.5s but I'm only adding to them if they are super cheap!
G4 As G3! Just trying to keep up with the new ones, but I do collect brushables, prototypes, blind bags, plushies, EG and Funko (regular and blind box) so it feels as if I will never complete G4!