One thing that evolved over the course of the series is the the way magic attacks are used. In PSI it was a fairly traditional system - MP to cast various spells learned on leveling up. In PSII, that was replaced with TP and techniques, but was still basically the same thing. However, items that could cast important techs were added, rendering TP less useful later.
PSIII had set technique groups for everyone. No one ever learned a new technique by leveling up. However, you could adjust the power of techs at certain shops, making one weaker or stronger.
PSIV went back to PSII's style, but with fewer tech casting items. They also added skills, which were (at least for Rune and Kyra), the same spells used in PSI. Instead of MP though, they had a set number of uses that increased as characters leveled.
Which of those do you think was the best? The traditional, the traditional with item backups, the distributed techs, or the skill system?