Trample. A 15/15 will only do so much if it can get distracted by a 0/1 bird.
Seriously, trample has been making the difference between chump-blocks and game-overs since day 0, yet the only mention it got in this thread so-far was in a post about how nasty it is to combine first strike with deathtouch. As far as underrated goes, you got your winner.
And on that note, yes, deathtouch & first strike
do form a pretty spiffy synergy, but did you ever
wonder why there's not a single creature out there that combines
trample and deathtouch? Well, it turns out that "any amount of damage is lethal" is a bit
too ludicrous if you pair it with "any damage done to blockers beyond lethal damage carries over to defending player/planeswalker."