The Finnish madman behind Brick Experiment Channel built a crazy Googol:1 gear reduction using 186 LEGO gears of various kinds.

The build was inspired by Daniel de Bruin’s “Universe’s biggest gear reduction” and the rotation time for the last gear would take 52433879932503535381614991275498187972589101825233846406570841889117043121149897330595482546 years to complete once!
 
Formula for the gear ratio:

24/8 * 40/8 * 40/8 * 40/8 * 60/1 * 12/1 * 168/1 * (140 / 8 + 1) * 141 * 20/12 * (40/8)^20 * 20/12 * (24/1)^20 * 56/16 * (36/1)^10 * (40/1)^18 * 15/9 * 56/1

List of gears used:

27x Gear 8 Tooth [3647]
1x Gear Expert Builder 9 Tooth [g9]
6x Gear 12 Tooth Bevel [6589]
2x Gear 12 Tooth Double Bevel [32270]
1x Gear Expert Builder 15 Tooth [g15]
1x Gear 16 Tooth [94925]
2x Gear 20 Tooth Bevel [32198]
23x Gear 24 Tooth [3648]
10x Gear 36 Tooth Double Bevel [32498]
49x Gear 40 Tooth [3649]
2x Turntable Large Type 2 [48452cx1]
1x Turntable Large Type 3 [18939 / 18938]
8x Gear Rack 11 x 11 Curved [24121]
1x Gear, Hailfire Droid Wheel [x784]
1x Gear Worm Screw, Short [27938]
51x Gear Worm Screw, Long [4716]