Sisays great for decks that want to assemble a particular combination of legendary cards, like Gaea's Cradle, Nykthos, Shrine to Nyx, or Elspeth, Sun's Champion
Being able to (potentially) cheat out cards like Worldspine Wurm, Shriekwood Devourer and Thunderfoot Baloth is nice, but there are some solid reprints, too
The main thing to remember is to find cards that have synergy with each other to help you achieve your win condition
Whenever a Desert card is put into your graveyard from anywhere, create a 4/2 green Plant Warrior creature token with reach
To see an up to date list of some of the most valuable cards check out TCG Player's list of the best selling cards of the week
This design is whats commonly referred in Magic: The Gathering terms to as a 'mana dork' - a creature whose sole job is to sit there and churn out mana for their controller to amass