Ok since we are talking about pve, Cleric damage is probably the best in game vs undead, even wizards would have trouble keeping up with the Smite line of spells, necro/pal/sk same thing , since they all get Smite spells.
So clerics meet all the requirements for a good tank.
1. Good defense
2. A way to keep aggro (healing)
3. Melee oriented spells/etc. (stun, hammer pet, str buff, self buffs etc, reverse ds)
I dare say clerics are probably best Defensive melee class in game, 2nd only to Warriors, but warriors cant heal, so probably makes clerics better for small group. Raid you want warriors tanking.
And at high level they get descent Nukes with good resist check (non undead nukes) so can deal out the damage.
Are we playing the same game?! Even with thier smite spells thier DPS is abysmal to say the least undead or not they wouldnt even rank top 5...
You dare say wrong plate armor does not make a good tank there is mitigation, discs and AAs that pallys SK's and warr's get that clerics never see. just because they can heal themselves dont make them a good tank that makes them a mana sponge.
Clerics are very tough but to even call them a real tank class is silly, They couldnt hold aggro to save thier lives (Healing dont generate the aggro you think it does) on top of the constant downtime there would be from them having to constantly heal themselves.
And if everything you said was even true your looking at it the wrong way everything you listed as a plus for clerics should be listed under pallys because of the fact they have better mitigation more HP's and AA's that make a tank well...a tank. Lastly if you really think clerics are that great of tanks you have not seen some skilled SK's a good SK with the gear and AA's are very impressive to watch between the ability to tap tank, pull and snare that with the mitigation and AA's trumps the cleric as well.
Besides the question was "whats best solo melee class" clerics are definately not that.