When working out the cost for a card with X, first decide what X will be, and then apply any cost reducing/increasing effects.
If a card costs G(X) (eg, Hurricane) then if you decide X will be 6, then Krosan Drover makes it so you only need to pay G + (4). If you decide X will be 0, then you only need to pay G, but you could decide X is 2 and still pay only G. The point is you need to declare what X will be.
There is no best card. It all depends on how you play and your preferences. In terms of power and toughness, it's the 13/13 Krosan Cloudscraper.
There is not just one most powerful card. Throughout the magic years there have been 9 cards that surface as the most powerful and the most feared. They were all printed in Alpha and I really Really hate them (just because I dont have them)the Power 9 are:Mox JetMox SapphireMox RubyMox EmeraldMox PearlBlack LotusAncestral RecallTime TwisterTime WalkThese cards are worth hundreds of dollars a pieceAnswerThe 'power nine' are notably more powerful then nearly anything that came after. However the card 'Black Lotus' is generally considered to be the most powerful.