My observation: the difficulty of the three keywardens are not even close to equal. In my opinion, the act I keywarden is designed to be neutral against both melee and ranged, the act II warden is designed to be anti-ranged, and the act III warden is designed to be anti-melee. Next, being a Demon Hunter does in fact bias my opinion on which is the strongest but I have several key points to discuss.
Act I warden: Molten, Teleporter, Jailer, Triple Fireball Attack.
Act II warden: Missile Dampening, Vortex, Teleporter, Mortar, Reflect Missiles Attack.
Act III warden: Frozen, Teleporter, Illusionist (removed), Fast (removed), Ice Breath Attack.
Observation 1) Why does the Act II warden have more abilities than the others?
Observation 2) With the recent nerf to the Act III warden (reduced damage, removed Illusionist, removed Fast) why wasn't the Act II warden nerfed?
Observation 3) Reflect Missiles does full damage (attenuated by Armor and Resistance) which is not like Reflect Damage which only reflects 10% of the damage done. Example: Bola Shot > Volatile Explosives hits me for 25k+ per bola reflected after Armor (4k), Resistance (450), SP>Gloom (35% DR), Sentry>Guardian Turret (15% DR) and I have 170k dps. While I can dodge some of the attacks, dodging all of them is not feasible with all the other effects going on (Vortex, Mortar, Teleport, and other enemies).
Observation 4) Not all classes have the same number of viable attack options for the Act II warden. Demon Hunters only have a few attacks which can penetrate the Missile Dampening: Evasive Fire, Multishot, Companion, Caltrops, Spike Trap, Cluster Arrow (non-Cluster Bombs rune).
Now for my personal suggestion: Blizzard, please consider adjusting some parameters for the Act II keywarden. The warden itself does not need to be damage- or health-nerfed however I feel that either the Missile Dampening or the Reflect Missile ability needs to be reworked, perferably the latter.