My main deck is a black/green infect deck. I know you guys are probably sick of seeing that, but it's so overpowered that I can't not do it. The way I'm building it right now (with the help of a friend) is to use groundswell and giant growth with this one equipment that I forgot the name of that has the creature it's equipped to do 2 damage every time it's the target of a spell... and gives it +2/+0 so I just equip it to a Plague Stinger and if all goes well my Plague Stinger turns out to be a 10/8 flier with infect... plus, with the damage I did to you while targeting him, there's no way to stop him. If I'm lucky, I can do that in four turns. And if not, I've still got good ol' Putrefax to deal 5 poison counters in one turn, but that's a last resort. Oh yeah, and there's Hand of the Praetors which gives other creatures I control with infect +1/+1 and gives a poison counter to an opponent whenever I play another creature with infect... look, there's pretty much no way to stop me. Even if you're playing red, I've got giant growths and vines of vastwood as well. And as for your creatures, there's that card that can give your creature -1/-1 and if it dies you get a poison counter. And at very worst, I'll pull out Phyrexian Crusader and gain protection from red and white and it's black so that means no lightning bolt, no doom blade, and no preventing damage or pacifism. And it's got first strike which, since it's infect, will affect your power as well as your toughness and it will do it before you even get a chance to damage me. The only way you could kill him is if you have at least a 4/3 green, black, or blue creature and if you do, I've still got my giant growths and doom blades.
Holy crap I went on rambling far too long. As you can tell, I'm a bit obsessed right now. I've only recently started playing in December, but I love it.
My personal favorite deck that isn't great but it is nice is my life gain/damage prevention deck. I'm talking about Soul Warden/Soul's Attendant + Ajani's Mantra + Ajani Goldmane + Ajani's Pridemates + Armored Ascension = Win. Then there's Serra Ascendant + Armored Ascension. +1/+1 for every plains I control and it has lifelink. And if I have more than 30 life, it gets +5/+5. So it's dealing about 10 damage every turn and it's flying and I gain life every time it attacks. And there's always Mirran Crusader to counter infect if I need to. It's not a type two deck so I can't use it in tournaments, but for casual play it's a lot of fun, even if it takes forever to actually get going.
My next idea was to make an infect/aura deck. Just put some aura gnarlids and give them infect with that one card that gives creatures infect. Some vines with a ton of auras spread across the board and then I'm hitting you with 10 infect counters and he can't be blocked. Just a thought right now, but my actual aura deck works pretty well when it's done right so I could give it a shot.
