White: Reveillark. This one's easy. Reveillark is amazing. Some of my runners up would be Karmic Guide, Rout, Serra Angel, Scars of the Veteran, and Blinding Angel. Other cool ones that aren't as good but that could be nice in casual decks are Zuberi, Mageta the Lion, Winds of Rath, and Radiant.
Blue: Force of Will. Blue has so many good 5cc cards that it's tough to choose just one, but Force of Will has been around for a long time and has ALWAYS been a huge influence on every format where it was present. Some runners up might be Tezzeret, Gush, Misdirection, Treachery, Future Sight, Dream Halls, Morphling, Evacuation, and Body Double. And some more cool ones are Thoughtcast, Scrivener, Scalplalexis, Traumatize, Time Warp (and Temporal Manipulation), Sunder, Parallel Thoughts, Mist of Stagnation, Ixidron, Cowardice, Bribery, Battle of Wits, Azami, and a bunch of other cards.
Black: Street Wraith. Yeah, I know. If that doesn't count, I'd go with Ad Nauseam. My runners up are Contagion, Hatred, Phyrexian Plaguelord, Living Death, Shriekmaw, and Call to the Grave. I also like Skirge Familiar, Yawgmoth's Agenda, Sengir Vampire, Haunting Echoes, Forbidden Crypt, Death Pits of Rath, Greater Harvester, and Conspiracy.
Red: Kiki-Jiki, Mirror Breaker. For some reason, most of the good red cards don't cost five mana. But I guess some runners up are Shard Phoenix, Covetous Dragon, Apocalypse, Torchling, Siege-Gang Commander, and Goblin Scouts.
Green: Saproling Burst. But I'm biased. Most of the good green cards in this mana cost range are big creatures. It's cool, but it gets a bit redundant. Some runners up I'd choose are Golgari Grave-Troll, Genesis, Overrrun, Arrogant Wurm, Bounty of the Hunt, Deranged Hermit, Gigapede, and Battlefield Scrounger. Maybe also Ageless Entity, Weatherseed Treefolk, Doubling Season, Voice of the Woods, Centaur Glade, Seedborn Muse, Restock, Plow Under, Endless Wurm, and Nature's Revolt.
Multicolored: Sliver Queen. The competition for in this one is surprisingly solid. Runners up are Sky Hussar, Spiritmonger, Mirari's Wake, Cadaverous Bloom, and Shivan Wurm. Some other cool multicolored choices are Sliver Overlord, Sliver Legion, Teferi's Moat, Frenzied Tilling, AEther Mutation, and Sek'Kuar.
Artifact: Memory Jar. Hands down. This is the best investment of 5 mana in the game. Other than maybe Ad Nauseam, Memory Jar is in a league of its own. My runners up: Panoptic Mirror, Stuffy Doll, Mishra's Helix, Leveler, Karn, Gilded Lotus, Razormane Masticore, Lich's Mirror, Lifeline, Puffer Extract, and Mirari. Some other cool 5cc artifacts are Eon Hub, Storm Cauldron, Thran Golem, Coat of Arms, Gauntlet of Power, Ring of Ma'Ruf, Belbe's Portal, and Dragon Arch.