Personally I felt that Hoenn was difficult purely due to the level. Both of my emerald cartidges were supposed to be legit but kept deleting themselves so I got plenty of practice - kept having to grind Victory Road to be able to compete. Gen1 gave me some trouble as did B/W however I had much more trouble with N than the Elite 4.
To me, it depends on my team, not on the Elite Four themselves. Pretty much all the Elite Four challenges I have faced have taken my many times to conquer. Platinum's Elite Four has kept me the busiest as of yet. The only time I hever ran through the Elite Four without dying was in my White game. but in my Black game the Elite Four is challenging my team to the max. So, like I said, it really depends on the construction of my team.
I find Black and White easiest because for the sole fact that you receive a Lucky Egg automatically, and when that is applied, I personally matched the levels of the Elite Four without actually facing them. The rest (from easiest to hardest) are:G/S/etc., D/P/Pt, R/S/E, and Red/Blue/etc.