Efficiency can be determined simply by the reward you receive at certain mana costs (Counterspell and Cancel do the same thing, but Counterspell costs less), the total payoff you receive from a single card (Hull Breach and Decimate are both good cards, but Decimate usually covers much more ground), or the payoff you receive in conjunction with other cards (Anointed Procession makes other token production cards completely absurd if unchecked, which is usually better than adding one more card that simply makes tokens like Spectral Procession in a token strategy)
With all of that said, I am sticking with Olivia as our commander here
There are several great places for buying cards: Local Game Stores
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Several cards in the set refer to modified creatures and reward you for making your presumably already awesome creatures awesomer
Nicol Bolas, the Ravager Just because a card is the obvious #1 doesn't mean it's not #1