In game? Probably Guardian, they seem like they're faster/more agile (double jump, blink, lift, glide, while Master Chief doesn't get any of that unless he has a jet pack). The abilities would definitely help the Guardian too. That's only in-game abilities though. Lore wise, Chief would win. But in the games Chief dies pretty damn quick, can't catch rockets or move 60 mph or one hand a sniper and shoot the pilot out of a banshee, so I'm going with Guardian there.