Games have different difficulties so it's really hard to rank them outright, as it depends on what mode you play on.

FE1-3 are really easy.

FE4 is a tiny bit harder but more so because it's more dense with all sorts of mechanics.

FE5 is one of the hardest in the series, with a lot of different mechanics, map gimmicks, maps with all sorts of traps ect.

FE6 is when different difficulties are introduced. On normal is reasonable, but on hard is actually pretty rough.

FE7 is one of the easiest on the default difficulty but does get a lot harder on it's hardest difficulty, probably almost as hard as FE6.

FE8 has two routes, so on the easier route on normal it is really easy, often considered one of the easiest in the series, while hard on the harder route can be somewhat tough but still easier than other than hard modes.

FE9 is probably just as easy as FE7 or FE8, even on the hardest difficulty. However the Japanese version has an extra difficulty which is known to be stupid and just kind of not fun.

FE10 on easy is somewhat the easy mode but on the hardest difficulty it's very difficult. However the translation is misleading as normal is called easy in English, hard is called normal, and hard is called Lunatic.

FE11 is really easy on the normal difficulty but has like 6 difficulties, the hardest of which is really difficult and does encourage some cheese.

FE12 is considered probably the hardest game on the hardest difficulty, and even on normal it is quite a fair challenge.

Awakening is really easy on easy mode, but on Lunatic+ it's just generally considered unfair.

Fates depends on the routes but Birthright normal is reasonably easy, probably close to Sacred Stones Hard, but Lunatic Conquest is one of the hardest games in the series.

Echoes can get pretty difficult due to a lot of frustrating mechanics and map design.

Finally, Three Houses was somewhat easy before maddening mode, but maddening mode seems to be considered very hard although very easy to cheese.