First of all... limiting this to just 10 is so hard, there are so many awesome cards out there, but these cards have resulted in the most amazing moments in magic i've ever experienced:
Deathrender - Probably the funniest/meanest card i play, its one of those cards I throw into every deck. It's the key card in my favourite EDH deck and provides some great drive for aggro decks. Turn any weenie into the biggest beef-stick you have or trigger any number of crazy combo plays (my favourite so far is sacrificing some random chump to play
Sidisi, Undead Vizier, having her exploit herself to pull a
mystic snake out of my deck and dropping that bad boy all in response to an opponent's field-wipe.)
Fog - There is nothing, and I mean nothing, like watching an opponent build up hoard of beef tokens, having him swing all out... and then telling him no. I once stopped over 3,000 damage with this and it felt so good.
Omnath, Locus of Mana - Lovingly referred to as 'Omnom', this sucker is the prettiest mana bank and my all time favourite card through my early days in Magic.
Thromok the Insatiable - Talking about rapidly ramping creatures; the exponential gains you get off Thromok are just too juicy. The look on people's faces when you drop a 9-billion/9-billion are just priceless!
Silklash Spider - I love spiders, they just hate flying so much, and this one is the absolute best... I once played against a zombie deck that dropped 200 something tokens, and then the guy thought it'd be a good idea to drop
Akroma's Memorial. I slapped a
Basilisk collar on my Silklash and watched the poor guy cry.
Glissa, the Traitor - One of my all time favourite abilities, deathtouch, paired with it's best friend, first-strike, plus a built in mechanic for pulling back that
Viridian Longbow your opponent so desperately wanted to rid her of.
Lumberknot - My other favourite ability, hexproof, keeps this tree rooted to the battlefield while it racks up those +1/+1 counters and waits for it's chance to punch faces.
Ink-Eyes, Servant of Oni - More an exemplar of the entire line of ninja cards; the ninjutsu ability is almost as annoying to deal with as control decks. There is just so much joy felt when you attack with a 1/1 unblockable and your opponents look at you with resignation in their hearts, asking how much damage they're really taking.
Chasm Skulker - I built a nearly fully functioning deck when I drafted M15 with this as the core card.
Aggressive Mining,
Jalira, Master Polymorphist, and a
Stormtide Leviathan for good measure.
Guttural Response - The best counter spell I've ever seen.