Some of these aren't the best cards you can hope to open, and some of them are. But most decidedly, these are my favorite cards to play when I make a green deck in M13 Limited.
This one is an easy choice of course. But the fun level is just undeniable. I mean two good sized creatures and 5 life all in 1 card? Yes please. Every time you play this card your opponent will immediately say "mother @#$%er", whether it is out loud or in his/her head. And don't get me started with how amazing this is when you have my number two most fun card...
Is there anyone in the world who does not love playing with Rancor? The answer is no. Not only is the card powerful as all get up, it is crazy fun to play with. All you need are the right (cheap) creatures, and tons of them. Perhaps a few ways to gain some card draw wouldn't be bad. I have yet to have Rancor in my opening hand and lose that game. Just wach out for instant speed removal and the dreaded...Unsummon.
5. Fungal Sprouting
Now remember, I said most "fun" cards to play with. On it's own it's not very good. Even with a bunch of 4/4 or 5/6 creatures it is not a high quality deck to play with. But is it fun? Hell yeah!! I don't draft these high but if one falls to me later on in a draft and I have some large creatures and a Titanic Growth or two, you can bet the house I will snap pick this over the other late going green cards in drafts. Making 7-9 1/1 creatures is not bad for a comined 6 mana. Also, chances are you killed an opponent's creature, or snuck in for some damage with your Titanic Growth infused Primal Huntbeast or Roaring Primadox prior to casting this baby.
Next, I will divulge my choice for the most fun Red cards. And soon enough, Return to Ravnica will be playing online, as well as IRL. Stay tuned for more lolz to come...