What Is the Best Item Active in the Game?

Zimmerman's powerglass

Duh

probably zhonyas

Mikaels Crucible. Removing a 3 second Morg snare from your front line can completely turn a losing situation around.

zhonyas > qss > red smite

Spooky Ghosts can be used offensively, defensively, or for scouting. Just a powerful mechanic.

Probably one of those.

simply because I once got a kill with the expiration explosion.

Hextech Gunblade. A low-cd (30 second) snipe of magic damage that slows, and synergizes with Hextech's passive being that you heal 15% of that damage.

Tie between and /

2.5 seconds of complete invuln, and a total cleanse.

one of these

zhonyas, maybe youmus or skirmersher saber.