Tough decisions...
Final Fantasy 7 is on the list. It was why I bought the system and it was a pivotal game in history for its plot and change in animation style.
I did enjoy the Breath of Fire games on the PS1, as well as the ones on the snes. I have no good reason behind why. I adored the atmosphere in the second game and was bent on collecting the others, which all have their own charms. Though the fourth installment has a terrible ending.
I was part of a Monster Rancher cult following back then. Something about putting random music cds in to make monsters was ingenious. And the quests! Oh what fun.
Then a fourth place tie between the Wild Arms series (or the first two thereof) and Valkyrie Profile. Both the series and the single game were good rpgs, though drastically different in flavor.
On a latter note, I'm sure I would have enjoyed the Spyro series back then. Sadly, it was one of a few games that made me violently ill. Tenchu 2 was also not part of the top four because of that reason. I can't justifiably rate a game if I could not finish it (excluding Battle Toads)
Edit: oh wait. I'm supposed to state why as well? I may as well edit that in then.