In terms of having been built by man.
I’m going for the helicopter.
Define best.
I’m struggling to imagine how the helicopter makes it into the top 10.
How complex are we talking?
If simple tools count I’d say…
Wheel
Plow
dam
knife
forge
etc…all modern things spring from these
If it has to be complex you have to go;
Mill
Steam engine
Locomotive
ICE
etc…again, necessary to move forward
If we’re talking recent, the PC has got to be a contender.
I’m confused by the helicopter also. It’s one of the most unstable machines I can think of.
Maybe some kind of qualifiers should be tossed into the question. I was thinking bicycle or computer.
Linotype machine: really complicated, aided mass publication, and exciting to operate, as it could blow hot lead onto the operator’a lap
I’d say it’s a tossup between the lever, the screw, and the inclined plane.
1x vote for my 1984 BMW R100 motorcycle. It has brought me so much enjoyment over the years, that it stands leagues above any other machine I’ve owned. It is a standard-style machine with good luggage capacity, so it’s comfortable for long trips. It has sporty rake/trail numbers that, coupled with the ultra-torquey 998 cc boxer engine, allow it to be ridden as hard as many sport bikes. And she also has the large tires and ground clearance to allow for dirt and gravel road adventures.
Maybe that’s not what you were going for, so i’ll suggest my home’s central air conditioning system.
Can’t really define best. Think it means the coolest.
Personally, motorcycles would be high on my list of all-time greatest machines. I think they do a lot of things for a lot of different people.
Practically-speaking, and in terms of farthest-reaching and bang-for-the-inventive-buck, I’m not sure where the motorcycle would rank. But i feel it has had a significant impact on both transportation and recreation, and therefore it shouldn’t be overlooked.
Then its the refrigerator. 
Bicycle.
Printing press.
The Steam Engine in general.
I don’t know if “The Electric Circuit” counts as “a machine,” but if it does, it’s on the list.
The simple Lock & Key has made some huge changes to human society, mostly for the better.
How about Surgical Tools in general?
ETA, just 'cause I’m feeling randy: The Sybian Machine.
I’m thinking of the Saturn V rocket with Apollo 11 atop it, LEM inside it, going to the moon, touching down, and coming back.
Or, the SR-71 Blackbird. Awesome jet, still very fast (although all (most?) are retired), and that’s 1950s technology.
Hey, a poster after my own heart! This post brought a smile to my face, thanks. It reminded me of my 1983 R80RT, which was an awesome machine. When I bought it, it had 60,000 miles on it. I put 80,000 miles on it and it was running strong when I sold it.
A smooth, smooth bike that carried me for many miles.
Large electrical generator.
Without massive power, none of your devices (post 17th century) could be built.
Dunno if that’s the “best,” but, by golly, it was one of the way coolest EVER!
CERN’s Massive Hadron Collider is another super-duper cool machine!
(You do not know how hard I had to fight with myself not to engage in the infantile joke of “Massive Hardon Collider.” Oops…)
Best? Probably the printing press.
Coolest? Well yes, I’m a diehard railfan, but the steam locomotive. Those bastards are ALIVE. They have fire in their bellies, they breathe, sing, scream, and sweat. Their various systems of water, heat, and steam seem almost analogous to biological ones. Different locomotives of the same design and manufacturer will steam differently, and an individual locomotive will have good days and moody days. Closest thing we have built to a living being IMHO, Turing test or no.
You’re late to the party. This, uh, inadvertent misprint has been published many times. (One wonders how many cases were really inadvertent.) Here’s one famous case.
Paddle ball.
Another vote for the bicycle - most popular, most efficient, and (IMHO) most fun form of transport in the world.
They got things right early with the bicycle, giving it a timeless quality. The bike I just rode into work on is the same one my great grandfather would have rode 100 years ago, in design terms. Mine is built with better quality steel and rubber, but take a step back and it’s the same machine.
I go for this wind-powered sand beast!