Nothing is more important in Magic than reliably playing lands and guaranteeing you have enough mana to fuel the later stages of the game, and these versatile cards keep you from falling behind
Sadly its limited to once each turn
You're already aiming for in your mana pool, so Two-Headed Hellkite is well worth it, and both flying and menace make it easier to attack with less likelihood of being blocked
Untapping any permanent has a dazzling array of uses, from netting you extra card draw with The One Ring to setting up combos with Gaeas Cradle and friends
After all, valiant triggers only once each turn and you need to constantly target it to make it grow, which means tons of mana and spells that couldve gone to cheaper targets
Iroas, God of Victory and Dolmen Gate are the best options