Other contributors have said "What is the best bicycle?" is the same question as "Which is the best bicycle?" If you believe that these are not asking the same thing and should be answered differently, click here
Bike size depends on your size, but in general terms, if you straddle the main horizontal bar of a bike, the bar should be an inch or two below your crotch.
There is no "best" material. They all have different characteristics, so what's "best" is determined by which features you prefer the most. Steel can be a bit heavy, but tends to be very long lived...
Difference between different lubes are quite small. You want to stay away from wd-40, 3-in-one, lube for chainsaws and motor cycle chains. The former are too thin and runny while the others are too...
"Gear" today can mean two different things. In engineering/tech speak a gear is a gear ratio, a way of trading speed for strength or the other way around. "Gear" in everyday language often means...