for me my overall favorite is definitely Gifts Ungiven. it's just so versatile and the decks around it can look so different depending on the plan. i think it's the ultimate deckbuilder's pet card, and i'm a brewer at heart.
in cube it's really tough because my favoritisms of cards were formed on a history of constructed. i'm tempted to say
mirari's wake, but that might just be because of how often it saves my ass in modo cube. i think i'd have to go with
opposition. it just closes out games with such ease, it's a pleasure to build around, and it's a posterboy for one of my favorite maxims i drill newer players on of ' you really need to be able to disenchant in some fashion if you want to win'.
constructs is just gifts ungiven again actually because of the radical effect it has on deckbuilding
i'm a little surprised with the anser i came out with for limited.
kor hookmaster. trip zen and zzw together were probably my second most drafted format ever. i think it's the best limited they've had since original ravnica. i know many people didn't like it for its extreme speed but i found it to be very rewarding of good drafting and fun to play. hookmaster was the second best card for the white (obviously aggro) deck because it was effectively removal because of the speed of the game, and came at a great price with a good body. (
brave the elements was the real blowout in white though)
for cube archetypes its wildfire fore sure. i almost never get to actually build it though because sometimes people just take wildfires and never play them because most people have never actually played a proper wildfire deck. reanimator/show and tell is a close second.