Top 5, In no particular order.

Smugglers Copter. Card selection, evasive, immune board wipes, gives all your creatures psudo-haste. All this for only 2 mana.

Tireless Tracker. Ultimate value. Either you deal with it straight away or it will bury you with card advantage, not to mention becoming a huge beater.

Gideon, Ally of Zendikar. Best walker, hands down.

Sphinx's Tutelage. Single card engine. Is key card is several archetypes.

Scrapheap Scrounger. Strong, recursive beater. Gives aggro and midrange a late game against control.

(dis)Honourable mention, Mwonvuli Acid-Moss. The worst feeling in the world was being on the draw and being acid-mossed turn 4. Have fun with all your 4 drops rotting in your hand. This card was so oppressive it caused me to quit for a while.

Not Top 5 Cards, but starting with 5 I reaaally like playing with (and deserve spots in top 100 cards). More to follow...

Valor in Akros

Drake Haven

Kari Zev, Skyship Raider

Insult // Injury

Enigma Drake

Thanks again for all the votes!

I promise I will update the OP by this time tomorrow, then I'll edit this post into my personal list starting with... Esperzoa!

Top 11 IMO, in no particular order:
Fatal Push and Smugglers Copter are by far the most used cards in the game and are in a league of their own (I mean can you find a deck with black mana sources that doesn't run fatal push or an agrro deck that doesn't use copter?

Liliana the Last Hope and Gideon Ally of Zendikar might be the best turn 3 and 4 drops and will often see play regardless of scheme because they represent so much versatility and can win games on their own.

Anguished Unmaking is by far the best form of removal in the game.

Winding Constrictor combos with more cards in duels then any other synergy based single card, making so many variations OP unless its removed.

Scrapheap Scrounger is the most consistent, fast, cheap and versatile headache in the game.

Yahenni's Expertise is the best sweeper in the game, considering it evades indestructible and doesn't slow down your pace with its ability to cast a free spell, and 9 times out of 10, swarms don't have toughness above 3.

Thopter Engineer is the best artifact token / creature / vehicle partner and will often win you games one turn earlier due to its cheap cost, sufficient toughness and of course the board haste mechanism it bestows.

Exquisite Firecraft is the best burn spell in the game and will often act as a kill spell vs blue control due to its, most likely, uncounterable nature.

Tireless Tracker is an almost automatic free spell because of the extremely high likelihood you get a clue the turn it comes into play. after that, if not dealt with it single handedly will account for both card advantage and a win condition.

Remove basic lands from list. Add:

Westvale Abbey

Wandering Fumarole

Shambling Vent

Lumbering Falls

Hissing Quagmire

Aether Hub

Evolving Wilds

Once more than 100 cards have 2 votes and the basic lands don't then they will be off the list.

Sorry I didn't update the rankings the other day, I will do it ASAP.

Edit : The OP is up to date, we almost have 100 cards on the list now! We need your votes to make the list more accurate, be sure to mention your favorite cards in this thread to get them higher up on the list! Is your favorite card better than an island? Prove it with your vote!

Also, please double check my work. Keeping this list up to date takes a lot of work and I'm worried that I might have made some mistakes along the way, any corrections are appreciated!

I never got to play with Mwonvuli Acid-Moss or that Mir card. Interesting they would put them in duels, I don't think they were even in origins or BFZ right? Acid-Moss seems like it would be good vs midrange or Ramp. Suprised the meta didn't switch to full on aggro if it were played everywhere. I guess it took the looter scooter to make the meta shift.

My favorite cards to build around or that just made me happy to play:
Sigil of the empty throne
Anointed procession
Torrential Gearhulk
Winding Constrictor
Ishkanah (i really like delirium cards in general)
Tamiyo
Angel of invention
Fiery Temper
Key to the city
Dusk // Dawn

Cards that I find myself putting in every deck that plays the colors:
The looter scooter
Beefslab supreme AoZ edition
Ajani
This is SPARTA
Unlicensed bolt to your face
Aethersphere "suck it aggro" Harvester
Tireless kill me now or get buried in cards
Censor
Lang yo thangs
Harnessed zap

Honorable mention to Thalia, she is so good and I cut her in my white decks too often.

Neither Perilous Myr nor Mwonvuli Acid-Moss were in Origins or BfZ, but they were in the starter box for Duels until SOI.

The meta during ORI was pretty diverse as Ramp lacked good finishers and payoffs for having many lands in play; stuff like Primal Bellow, Chandra's Ignition or Woodland Bellower was good in midrange as well, the only Fireball-alike card was Ravaging Blaze and Into the Maw of Hell as LD spells #5-7 were not enough to make LD potent enough.

Then BfZ rolled around, and suddenly Ramp got the payoffs it needed - Ulamog, the Ceaseless Hunger, Omnath, Locus of Rage, From Beyond, Greenwarden of Murasa just to name a few. And yes, for the most part the BfZ meta consisted of GRx ramp and mono-Red aggro, with occasional Thopters and Crats mixed in.

It took a Mwonvuli <-> Explosive Vegetation exchange in the starter box as well as good, cheap exile removal (Declaration in Stone, Anguished Unmaking) from SOI to make the meta shift.

I think cards that have been pulled from the game shouldn't be on a list of best cards in the game fwiw. I'd cut those once list gets to 100+ (maybe do honorable mentions for them?)

Anywho, more recs - getting to cards already mentioned by others

1 x Declaration in Stone
1 x Blessed Alliance
1 x Glory-Bound Initiate
1 x Collective Effort
1 x Always Watching
1 x Bygone Bishop
1 x Eldrazi Displacer
1 x Gisela, the Broken Blade
1 x Angelic Destiny
1 x Sram's Expertise
1 x Gideon, Ally of Zendikar
1 x Cast Out
1 x Planar Outburst
1 x Archangel Avacyn
1 x Bruna, the Fading Light

1 x Thing in the Ice
1 x Jace, Vryn's Prodigy
1 x Censor
1 x Telling Time
1 x Unsubstantiate
1 x Pull from Tomorrow
1 x Pieces of the Puzzle
1 x Broken Concentration
1 x Scatter to the Winds
1 x Spell Shrivel
1 x Glimmer of Genius
1 x Jace, Unwraveler of Secrets
1 x Disciple of the Ring
1 x Torrential Gearhulk

1 x Blossoming Defense
1 x Oath of Nissa
1 x Sylvan Advocate
1 x Druid of the Cowl
1 x Grapple with the Past
1 x Reclamation Sage
1 x Tireless Tracker
1 x Nissa, Vastwood Seer
1 x Nissa, Voice of Zendikar
1 x Explosive Vegitation
1 x Nissa, Vital Force
1 x Verdurous Gearhulk
1 x Ishkanah, Grafwidow
1 x Rishkar's Expertise
1 x Gaia's Revenge
1 x Sandwurm Convergence

1 x Harnessed Lightening
1 x Cathartic Reunion
1 x Collective Defiance
1 x Fiery Temper
1 x Exquisite Firecraft
1 x Kozilek's Return
1 x Radiant Flames
1 x Hanweir Garrison
1 x Thopter Engineer
1 x Pia Nalaar
1 x Pia and Kiran Nalaar
1 x Chandra, Torch of Defiance
1 x Glorybringer
1 x Chandra, Flamecaller
1 x Combustable Gearhulk

1 x Fatal Push
1 x Grasp of Darkness
1 x Gifted Aetherborn
1 x Murder
1 x Yahenni, Undying Partisan
1 x Nantuko Husk
1 x Read the Bones
1 x Liliana, the Last Hope
1 x Haunted Dead
1 x Kalatis, Traitor of Ghet
1 x Languish
1 x Yahenni's Expertise
1 x Ob Nixilis Reignited
1 x Liliana, Death's Majesty
1 x Ever After
1 x Noxious Gearhulk
1 x Never // Return

1 x Reflector Mage
1 x Ajani Unyielding
1 x Anguished Unmaking
1 x Ayli, Eternal Pilgrim
1 x Sorin, Grim Nemesis
1 x Veterin Motorist
1 x Honored Crop-Captain
1 x Nihiri, the Harbinger
1 x Fevered Visions
1 x Stormchaser Mage
1 x Bounding Krasis
1 x Kiora, Master of the Depths
1 x Forerunner of Slaughter
1 x Olivia, Mobilized for War
1 x Unlicensed Disintegration
1 x Arlinn Kord
1 x Samut, Voice of Dissent
1 x Voltaic Brawler
1 x Tamiyo, Field Researcher
1 x Cut // Ribbons
1 x Heaven // Earth

1 x Walking Balliata
1 x Bomat Courier
1 x Smuggler's Copter
1 x Key to the City
1 x Scrapheap Scrounger
1 x Alchemist's Vial
1 x Chief of the Foundry
1 x Filigree Familiar
1 x Aethersphere Harvester
1 x Thought-Knot Seer
1 x Reality Smasher
1 x Skysovereign, Consul Flagship
1 x Distended Mindbender
1 x Ulamog, the Ceaseless Hunger

I was thinking of starting a thread like this, but more like "top 5 cards for each colour" - The cards that are just plain good regardless of the deck they go in. (i.e. the 5 most Baneslayer Angelesque cards)

Possibly make it a non-Planeswalker list since some like Gideon and Lily are just friggin' obvious.

Any interest?