I wouldn't say WipEout HD or Warhawk are the hardest.
WipEout HD sort of takes a simple learning curve, I used to think this game was hard too, but it actually gets pretty easy when you know where to barrel roll, I've given a few of my friends advice on this game, and they got good at it in no time.
Warhawk isn't necessarily hard at all, it just takes a lot of effort. Present day it might be hard for newcomers to get their Air Badges/Medals because most of the community that still plays uses LG as their primary weapon. But this game isn't hard at all, its completely accumulative; all the badges, medals, and even the ranks are basically given over time, given that you use all the weapons persistently. The Platinum revolves around reaching General, and getting all the Badges/Medals (Considering that everything else can be done in a private match). Each award has a Global & IAR requirement to get them. Once you achieve the the Global requirements (accumulative portion) for any award, you do the IAR (In a Round) requirements once, and you get each award just like that.