Let's see,...
from the perspective of being the "hardest" due to a pretty steep learning curve + some unclear, awkward, and unintuitive tutorials/explanations - in 'that' way -
Cross Edge, for the PS3 might be the 'hardest' game that I've platinumed,
from the perspective of huge amounts of content + a big time-commitment -
Disgaea D2: A Brighter Darkness' platinum would probably qualify,
in terms of an outrageous difficulty spike + a pretty tough post-game dungeon that *must* be completely beaten on the highest difficulty level for the platinum;
Tears to Tiara 2: Heir of the Overlord, gets the nod,
the most "grindy" kind of hard platinum that I got is probably
Class of Heroes 2G,
IMO, the hardest one that I have *not* ever gotten back to, to finish-off the platinum =
The Guided Fate ParadoxI guess that those might be my current "Top 5".
To mix-it-up a bit...what are some of your *
Easiest* games that you've all platinumed and/or got 100% of the gamerscore?
Mine would most likely be
Last Rebellion...
...not only is it the only RPG that I've ever seen that can be platted in only 1 or 2 days of (3-5 hours of gaming per day)
*BUT*
it's also the only one that I've come across with more golds & silvers than bronzes