In terms of looks, lore, and even how much power it gave you at the time how do you rank the legendary items? Kael'thas items and such don't count.
Warglaives of Azzinoth - among the most dramatically awesome looking weapons, I mean they were the blades that Illidan had the audacity of brandishing in front of you and yelling "You Are not Prepared"......"well I got your weapons bra guess I came pretty stacked" They did wonders for the gearcheck against Brutallus in the Sunwell.
Thunderfury - still the largest one hander in the game, it was an item so powerful that it still had it's uses at level 70. Having to kill the Prince Thunderaan made it feel special, if there was more lore behind it though it would be my number one due to it's power, but Illidian is Illidian sorry.
Val'anyr - not to familar with the background of this one but it get's points for being a healing legendary. It also did wonders for Heroic guilds to be able to shield their allies. I'm not sure however if this Legendary ever became a victim of the high item inflation of the ICC epics.
Dragonwrath, Taragosa's Rest - Let me first say I'm being bias since I was around much for Cata to properly judge their legendary's, perhaps they should be higher. The quest chain for this was awesome, you see Malygos become to new Aspect of Magic, the capital city filled with blue dragons with a silloute of Taragosa, and transformed you into a freakin dragon. I also heard that it made casters become extremely powerful. Looked amazing also.
Fangs of the Father - probably should be higher since their created from parts of Deathwing I believe. I have no idea how strong they made you, but I know they gave you batwings like awesome!
Sulfurnas Hand of Ragnaros - Not to sure just how much more powerful this item made you, but the name is just so awesome to me and back then Ragnaros was just such a badass having his weapon was just so awesome.
Shadowmourne - The one legendary I actually acquired during Wrath, to be honest though I always found the model underwhelming, while it's design reflecting Frostmourne was cool, the axe itself just seemed far too small, almost like a hachet. Those purple swirlies you got though made you look so freakin pimp though. It's power spike though I don't think was as dramatic as other legendarys.
Legendary Cloaks - well since everyone and their mother seems to have this it kind of makes it less awesome, however seeing the Celestials appear behind you is sweet. The power spike is quite incredible also.
Thori'dal Stars of Fury - by far the worst imo, absolutely no lore behind it, just a legendary the final boss of the expansion drops therefore it ruins the purpose of the powerspike you receive. Also have no idea how much stronger it made you.