The boost you get from countering a two or three drop and jumping to five or six on your next turn is a much greater swing than countering a four drop and then having 8+ mana for the main reason that you don't have that much useful to spend that kind of mana on very often and if you do it is not as significant as an earlier mana boost
Cultivate And Friends As one of the top five most-played Commander cards by EDHREC numbers, with 1.71 million decks running it, Cultivate needs little introduction
, , Discard a card: This creature deals damage to target creature equal to the discarded card's mana value
It is pretty nasty for the control player too although Duress stays pretty good against them throughout the game
$2.20 Rare-B-Gone Sorcery Each player sacrifices all permanents that are rare or mythic rare, then each player reveals their hand and discards all cards that are rare or mythic rare
You put, into play, d20 mana value creatures from all graveyards to your battlefield