Quick combo-breaker to the favorites discussion:
I'm making a Riku EDH deck, and I was wondering what some of the most fun red, green, or blue cards are. Combos of all of them, two or three color kinds are welcome, and more towards red/green than control blue are preferable. I just want to play crazy stuff and keep everyone wondering whats gonna happen. I'm running an Unexpected Results for sure.
Let's see... all my recommendations will be based on the way I build EDH decks (moderate powered, somewhat fair and fun effects, and mostly built for multiplayer EDH rather than duels), and they're quite a few... anyway, here we go!
Experiment Kraj: Does crazy things on his own, and if you have a small +1/+1 counter subtheme, it becomes even better.
The Morphling family (Torchling, Thornling): They're really versatile, but nowadays they aren't powerful enough to make everyone on the table instantly hate on you. Thornling gets to dodge most global sweepers unlike the other two, but is owned by targeted spells.
Alchemist's Refuge: This might get you hated on a bit, but not nearly as much as playing a Vedalken Orery. Yeva, Nature's Herald is another nice way to get sneaky without being completely unfair, if you decide to play mostly creatures.
Cytoshape: Fun shenanigans, even if it only transforms a single creature, and only for a turn.
Spitting Image: A copy spell which can be recast as long as you've got lands in hand to pitch. It's fair but still powerful.
Wild Ricochet: Versatile "Surprise!" card. Remember you can always give the spells with positive effects to a third player in order to make some friends.
Temporal Mastery: Really powerful miracle, but can't be recurred, so players are less likely to gang up on you just in case you start recurring it like with other extra turn effects.
Vesuvan Doppleganger: Repeatable, affordable copy effect.
Djinn of Wishes: Sort of like a miniature, three-use Unexpected Results.
Sphinx of Uthuun: Fact or Fiction on a hefty body. Play in a politically-correct way and you're likely to end up drawing five cards with it most of the time.
Erhnam Djinn: Great for when you want to team up against someone else who's running forests.
Capricious Efreet / Charmbreaker Devils / Moldgraf Monstrosity: Effects at random!
Research // Development: Seems like a fun split car you'd be able to run. I guess it depends on how your playgroup handles the Research side.
Signal the Clans: While I'd hate doing the usual "find three funcionally identical cards to ensure I get what I want", and am against playing tutors in the format, you can always search for things at random and see in what direction you're pushed by whatever you get.
Conjured Currency / Risky Move: Chaotic, but won't get player hating on you as much as stuff like Scrambleverse, Warp World or Chaos Grip.
Curse of Stalked Prey: A miniature bullseye you can paint on anyone. Be careful, though, because they probably won't be happy about it.
Hypersonic Dragon: More flash!
Commandeer: Not a fun card by any stretch, but can be useful as a trump card for when you have to get serious.
Terastodon: You have to be careful with what you blow up with it to avoid earning too many enemies, but it's a great card to have around (even better when cloned with Riku's ability).
Magus of the Vineyard: Acceleration for everone!
Exotic Orchard / Forbidden Orchard: Add colored mana. The Forbidden one is great for earning friends while fixing your mana!
Bloom Tender: Nice acceleration.
Plaxmanta: Versatile combat trick.
Varchild's War-Riders / Hunted creatures from the original Ravnica: Interesting political cards.
Simic Guildmage: Even if you don't use that many +1/+1 counters, the aura swapping ability can be useful, and quite surprising.
Oran Rief, the Vastwood / Novijen, Heart of Progress: Nice utility lands.
Zameck Guildmage: A Novijen that can be activated more than once, and can be used to draw cards as well.
Plaxcaster Frogling: You can protect your stuff or surprise opponents by sharing the counters with them to either protect them or prevent them from targeting their ownn stuff.
Trade Secrets / Echo Chamber: Very political cards which can be used to devastating effect in the right circumstances.
Pulse of the Grid: Repeatable card drawing which you're likely to get back most of the time.
Heartwood Storyteller: A great card if your deck's more creature than spell-based.
Hope that helps!
Ninja edit: all soulbond creatures can be useful as well, because you can pair them with an opponent's creatures to give them a hand. The best would probably be Wolfir Silverheart and Tandem Lookout and the other guy that grants haste.