Ooh, this is tough for me... I was so young when the G1 line ended, I have no memory of who was my last. @_@ Going chronologically, it would have had to be one of the ponies in Year 10, but IDK if Kimsites is chronologically correct! If it is, it would be most likely to be one of the Merponies, Baby Sea Princess!
However, I also got several G2 ponies when they came out, and even though I was a "pre-teen" at that time, I still gladly bought them whenever I went to the store, so maybe one of those would "technically" be my last? But when does 'childhood' end and 'teenagehood' begin...?
Maybe Princess Silver Swirl, who was one of my first/earliest purchases from the internet?