Here's a start. Stuff I have in my cube or have played in other cubes
White:
Hero of Bladehold: low curve Aggro/Weenie alpha striker
Gideon Jura: the most bad-ass plainswalker in the multiverse. Will eat your opponent's attacks for days, enabling alpha strikes or protecting your other planeswalkers/threats. Also a quick, typically hard to kill clock on an empty board
Sun Titan: pod finisher/reccursion is valued. Also good in weenie strategies with a higher curve than is suitable for Hero
Angel of Serenity: control the board/answer threats
Elesh Norn: crush creature decks under your oppressive AOE
Blue:
Jace, the Mind Sculptor: "
My Life Would Suck Without You"
Frost Titan: a tap out style where the body is relevant and the tapping ability helps push attacker through
Aetherling: a threat light kill-everything style where playing at instant speed might be important. Likely your only threat.
Consecrated Sphinx: 1-for-1 style deck where you plan to win on late game card advantage.
Sphinx of Jwar Isle: a slower but less mana hungry Aetherling
Black:
Grave Titan: Immediate 10 power on board to close games out or to protect your own board. Very quickly takes over a boardstate
Mikeus, the Unhallowed: card advantage finisher for pod/zombie/sacrafice strategies. Turns all your dudes into Vorapedes
Masacre Wurm: closes out games versus aggro strategies, can set up alpha strikes
Sheoldred, the Whispering One: Pox/Attrition strategie' late game closer. All great pod chains end here.
Griselbrand: on-color reanimator target of choice
Red:
Sulfuric Vortex: Once the mono-red player flushes the toilet, there is no escape
Hellrider: For more creature-centric aggro decks that don't have a burn-out finish
Thunermaw Hellkite: evasive, quick clock. Great in big zoo/aggro or as a midrage play
Inferno Titan: for bigger, badder red. Great in ramp as a mid to endgame card as well.
Greater Gargadon: a big body for some of the smaller decks, especially those with sac/Act of Treason themes
Green:
Garruk Wildspeaker: in weenie/zoo decks playing green, Overrun Ult can win games quickly. Especially with Lingering Souls.
Garruk, Primal Hunter: card advantage engine in green that either poops out dudes or draws you more dudes until you win
Thragtusk: finisher if the guy to your right is for sure mono-red
Wolfir Silverheart: makes two huge dudes, can end games quickly or help stall boards
Primeval Titan: the trample can win games on its own. For every other situation, grab yourself a Kessig Wolf-Run
Rampaging Baloths: landfall finisher that goes tall
Avenger of Zendikar: landfall finisher that goes wide
Multicolor:
Geist of Saint Traft: can close games in zoo or Delver very quickly
Sphinx's Revelation: win the card advantage war late, lifegain is valuable if you know the field is aggro-centric
Obzedat, Ghost Council: hard to remove body which can slowly grind out games.
Ajani Vengant: accentates any and all mana issues your opponents have to keep them out of a game. Helps stabalize early as well
Olivia Voldaren: given enough mana, easily wins creature fights. very vunerable to removal
Falkenwrath Aristocrat: quickly end games for B/R sac/zombies decks. Very glass-cannony without proper support
Knight of the Relequary: huge, cheap pseudo-card advantage engine for aggresive decks
Kiora, the Crashing Wave: the subgame of "can she unleash the Kraken?" wins games by exhausting opponents resources, or by unleasing the Kraken
Simic Sky Swallower: massive untouchable ramp/reanimator target
Cruel Ultimatum: cleans up boards, insane card advantage, and a lava axe all in one
Colorless GRBS
Skullclamp: GRBS
Jitte: GRBS
Batterskull: GRBS
Wurmcoil Engine: GRBS
Colorless:
Karn Liberated: giant threat that can pick apart a boardstate and clean up from there. Massive loyalty helps keep him alive long enough to do so.
Myr Battlesphere: provides bodies that roadblock non-evasive attacks. Makes for a quick clock turning sideways too
Kozilek, Butcher of Truth: dedicated ramp game ender. Is Opportunity on a stick, except that stick also wipes your opponents board.
I know this isn't an exhaustive list, but its a start. What am I missing?