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
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
There you can find decklists being shared, new decks being brewed, championships and metagame discussion (and the popular which card should be banned now rant as well)
If it enters with counters, those counters are included
The main thing to remember is to find cards that have synergy with each other to help you achieve your win condition
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