Dragon Quest I -- it gave us the slime.
Final Fantasy IV -- Though I loved the original, IV changed what I and many others have come to expect from RPGs. Couldn't go back to FFI, or DQI, etc., after FFIV. If VI or VII are superior, IV made it possible.
Shining Force I -- or, really, this might just be a placeholder; I don't know what really influenced the development of SRPGs, but I figure something must have, and that something deserves mention.
Everquest -- WoW before WoW.
ETA: Oh yeah, OP -- make sure when you start your elimination list to make clear to people that this isn't the "Best RPG of all time" list, but the "most influential"... otherwise, whatever their actual influence might merit, VI & VII will beat the shorts off of my FF IV. :)
