For my own stupid reference because I'm still trying to pull together nice transmog sets for my death knight:
Saronite War Battleplate which I already have since it's pretty much death knight starter gear. I don't really like it all that much though. But I'll include it on this list anyway.
Scourgelord's Frigid Battleplate of which I am only missing the pants and the helmet now (and the helmet isn't vital since I always hide my helm anyway)
Exalted Plate (recolour) is really cute but pretty old and might be even more of a pain to get since I think a lot of it is crafted and I don't have a decently leveled blacksmith
Doomplate Battlegear is also old (looks like it's all Burning Crusade dungeons) and fairly plain but... I still like it. I figure the whole dark blues/purples/blacks would look good on a death knight.
Abandoned Fealty Battleplate (recolour) which I REALLY REALLY like (except for the shoulders, the shoulders are way too big and goofy looking) but will be impossible for me to get because the dungeon it's in is all stupid jousting and is really hard to solo because of that. ugh.
This will take some doing to pull together but it's sexy as fuck Bloodscale Plate so cute~
Guide to Death Knight transmogs Outrunners Legguards I totally want to make a dwarf just to wear these
Roy G. Biv this is just rad
this one's cute and I have some of the pieces already I like this one too I'd like to have this on a caster someday