No, I think Red and Blue are the easiest. Emerald is probably one of the most difficult, especially if you include the Battle Frontier. The level jump between the last trainers and the E4 is extremely high.

But don't the final Pokemon-thingys have weaknesses? My friend managed to beat them when his best Pokemon was 15 levels lower than them.

You still need to grind 5-10 levels to reach 15 levels under the E4 level. That level gap is simply that high in RSE.
And if you read my previous challenge report, you can see that I beat the Red E4 with a team over 20 levels under the E4 level. A team filled with pretty terrible Pokemon at that. You will have gained a lot of EVs/Stat Experience by then and you can abuse type match-ups, giving you a pretty great advantage. So this goes for every generation.

I found this hack for pokemon Fire Red. It seems cool, but I'm no that far in to it, will update later.

http://www.pokecommunity.com/showthread.php?t=279049