The Question you have asked "Which Libram is best for tanking?" is slightly misleading. Currently, in game, there is no Tanking Libram that should be used 100% of the time for tanking every boss in the game. Librams are designed to be very situational, thus, they should be changed often depending on the encounter in question. Do not be swayed simply by item level and assume that the 245 Libram that provides dodge is magically the best Libram to use.
For the most part, and for all intents and purposes, the Librams you should use the majority of the time are teh 245 Libram that provieds 200 dodge and the 226 Libram that provides 450 BV. At this point the Libram of obstruction is mostly a mute point since it is bested by the 226 Libram. For a boss encounter that requires a high number of avoidence use the dodge Libram. If you are blocking often and want more consistency in your fight use the BV Libram. Personally, I choose to use the the BV Libram the moajority of the time. This is becuase most encounters are best suited for a consistency in damage that the BV Libram helps provide. The 213 Naxx Concencration Libram is still the best AOE tanking Libram available. Especially if you are tanking casters.
To choose your Libram; anyalyze the situation and determine which quality is needed most. As a Prot Paladin you should never have space in your bags

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