Might just be my bias but I honestly say Fire Emblem 7.
I never really liked FE8 [looking at you Eirika] and I got really annoyed at the easier difficulty due to the Tower of Valni. ... Plus, some of those monsters creeped me out back then. Of course, the story line was stable and who can't resist Ephraim?
FE6 was also good, but the holes in story kind of made it... iffy. Plus, this might just be me being petty, but I never really liked Roy's fragility, Lilina not being a lord and how the thieves couldn't promote.
And FE7 was a nice thing to have a tutorial prologue. The characters in the game were unique and all had connections. Let me tell you how many times I pulled a Kent whenever Sain flirted. Let me tell you the shrieks I conveyed at Uther and at Eliwood.
The music was definitely a plus. Contributed very well, and I always hate having to turn the volume off. I remember at one point I turned on my DS just to listen to FE7's Sound Test.
The maps also weren't ridiculous or too easy. It was a challenge that I fully accepted. Trying to attempt HHM on some of them though... -shudders-
All in all, I feel FE8's difficulty wasn't up to my standards of HNM. Plus, Ephraim's mode left a hole in the story when
At least in Eirika's mode, you find out why and even help him.
FE7 has more character development and the story was pretty great in my opinion. It has a stellar soundtrack and a nice diversity of characters. Plus, it holds the one axe-using lord before promotion!