Urza's Nine Titans. Because then I'd be a badass Planeswalker with near-unlimited power and probably centuries of experience under my belt... sure, my odds of survival are almost nil, I stand a better than 1/3rd chance of being killed by my own side, and even if I don't, I'm fighting Mechanized Pure Evil with a healthy dose of Agonizing Necromantic Deathfog Doomgod, and even THEN if I survive odds are the fractures in the timeline caused partly by our struggle will eventually see my own demise, likely in self-sacrifice... but dude! PLANESWALKER. I could freaking summon any of y'all to kick ass in my name! And Urza gives us ALCHEMICAL MECHAS to use! We get to blow stuff up with SOUL BOMBS! Greatest. Job. Evar.