Worst Lord in terms of design: CORRIN. I don't know what the fuck he/she's wearing but I want to punch them in the throat for it. Eirika for having the most forgettable design in the series looking like a female Marth before Lucina was a thing and her Gary Stu brother is worse at not looking like Marth. FE2!Alm and Celica are archaic to the point where the Old Mystery art looks better. Dark Dragon!Marth would be here but I enjoy that design like how people can enjoy The Room and Birdemic.
Worst Lord in terms of writing: FE10!Ike because I don't really know why he's given a third of the game that should have went to the new characters when Ike is being bland and boring as shit because the writers have no idea what the fuck to do with Ike because his story/character development ended in FE9. Honestly, I think that FE10!Ike is the second worst lord next to CORRIN. Speaking of which, FUCK CORRIN. Male, female, Conquest, Birthright or Revelations, CORRIN will forever be shit regardless of what route you take or what gender you make them be. Sigurd, for all of his badass-ness and reliability feels like all of his character traits are on cliff-notes and didn't really express any character because most of his conversations are there to flesh out the plot further. I blame the SNES/SFC's limitations so Game!Sigurd gets a dishonorable mention. Manga!Sigurd is MY headcanon and I'm sticking to it.
Worst in the lore: CORRIN, because literally everyone likes CORRIN except assholes (Iago and Hans), blob monsters (Garon), and people being possessed by evil dragon god abominations (Conquest!Takumi before he dies.) so they don't fucking count. CORRIN can't go anywhere without anyone praising them even as they constantly fuck up. The prime example of this is Hinoka in the ending of Conquest. CORRIN ravaged her country, killed her mother by proxy, killed many loyal soldiers and retainers, killed both of her brothers, took her little sister as a hostage among other things and she treats it like a slap on the wrist because it's fucking CORRIN. Hinoka literally has a pile of good reasons to want to have CORRIN'S head on a fucking pike. I hate when people use the term Gary Stu/Mary Sue for characters that they hate because most of the time it's not the case. When I mean the term Gary Stu/Mary Sue, the narrative itself will bend over backwards to convenience the character in question. Which, CORRIN has in fucking spades.