Creatures that can tap for mana then untap for less than that amount of mana can generate infinite mana, and reducing those activation costs are a big part of how netting mana each iteration is possible
Ral Zarek is solid for your decks, and I played him in my Standard deck that did some damage at FNMs at the time, as people would really respect Ral in that era, especially with the Double Lightning Bolt available for four mana at -2
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Although there are numerous ways to play and multiple expansion sets you can choose between, I'd recommend grabbing one of the many two-player starter kits if you're new to all this
A low opportunity cost mana sink that scales into the late-game is awesome, and Ill spoil the fact that the rest of the cycle did not make my list by saying that I believe Lair of the Hydra to be the best of the lot for Cube play
Nicol Bolas, the Ravager/Nicol Bolas, the Arisen Nicol Bolas, the Ravager is a new Transform Planeswalker