Since theres a pretty significant disproportion between colors, Im just going to run down the list alphabetically (you can tell Im not being biased with this particular organizational schema because a white card tops the list and theres no blue cards in the top three)
Commander: Aragorn, the Uniter
The borderless non-foil version is valued at around $16, and the regular Tri-land version of Sparas Headquarters is priced at around $7
My last two choices are so good, they were reprinted in the Doctor Who commander decks, but these cards are originally from Modern Horizons 2 and Amonkhet
It allows you to look at the top of your deck at any time, and you can play the top card of your library if it's a land and an opponent has more lands in play than you
Second, the cost reduction mechanic usually doesnt allow you to cast Karador super early (barring something like a Hermit Druid), but it does help you set up combo plays with creatures in your graveyard during the mid-late game
