I am one pony short of a US/Euro G1 collection [dang you purple symbol Baby Princess Sparkle your magenta sister needs you to come join us lol!]...and if I am honest there was little logic to my getting there
I started with "the ones I want most"
Then moved on to "at least two from each set"
Then moved on to "complete the sets I like best"
Then moved on to "complete all sets"
So little to no pattern! I tended to have 20 or 30 ponies at a time I was always on the look out for and as and when I found them I swapped them out for another pony to keep the list around the same number. Occasionally I would jump outside that list if I purchased from a seller who had other ponies for sale I was still looking for. As to who made that list - the priority was simply down to who I liked best at the time.
As for now I am at that upgrading stage where there are a few ponies I would simply like better versions of so I have the same kind of list. On top of that I also always have 4 or 5 accessory items which I am pretty desperate [always] to track down.