This is kinda rough, since I guess you'd have to like take into account everything you can do in the game. That is to say, if you make the right pairings in FE4 and get the Holy Weapons, then it's absurdly easy for the most part...however, you can still get nuked in the aforementioned Shanan/Patty scene as well as (IMO) the part with those the Falcon Knights with Earth Swords of ultimate pwnsomeness (iirc, they have Nihil, Critical, Continue, and good Leadership =/)
Then of course, you've got the difficulty of a first time runthrough, which I guess would be the most legitimate way to judge it, since you have no prior knowledge of what to do...so going by that, I'd say...
FE8: Just...never difficult...Ephraim is god, Eirika is goddess, and you have a ton of h4x characters and weapons
FE7: I never really found this hard, but since I didn't like the characters at all, I never got around to playing HHM (EHM was a joke, imo)
FE9 (NA Normal and Hard): I can't really remember a hard part in this game, but there are decent bosses and enemies with random skills (SM with Adept, wtf?)
FE6 (Normal): Not really difficult, but your characters aren't great, so meh
FE10 (NA Normal): Dawn Brigade is the hardest part, the rest of the game is pretty straightforward and easy
Now to the more ridiculous ones, imo
FE10 (NA Hard): Dawn Brigade ftl...the first time I played this on HM, I used Nolan and Eddie to basically smash through Part 1...on later runs, I just used Zihark

. Lack of weapon triangle is generally good, but in the beginning, it makes Eddie worse.
FE4: I'm going to go out on a limb here and say that on your first runthrough, this game can be a killer. You don't know where to get the pursuit ring, Hero's Axe, and other useful things, and you can randomly get nuked at certain points (the chapter that Levin arrives in comes to mind, since you probably won't get the Bargain Ring). In addition, depending on what you do with Julia, it'll be quite difficult to handle Julius in the end, even with the characters that you thought were so h4x earlier (why hallo thar Nihil, when'd you get there?!?!?)
FE6 (HM): Wtf...I've seriously restared chapter 1 like 4 times on a runthrough...Marcus sucks, and Alan/Lance randomly get hit by those 32% axes and get destroyed...
FE5: On your first runthrough, you don't know when you can get Stamina drinks, so you may stave off on buying them so you can save money and just buy them as you need them. That is probably a mistake. Also, if you don't know where to get certain promotions and crap, you miss out on better characters. Sleep and Silence being permanent is annoying, 99% hit chances missing is annoying, healers that wiff on healing is also annoying. Even though no one's stats look too impressive, I still like fighting with some of the random good enemies that you encounter. The boss doesn't live up to the rest of the game, but eh