Best Simulator?

Train Simulator, bitch!

Look for Digital Combat Simulator A10 Warthog, its a incredible detailed sim, even more realistic in some features than FSX.

The Sims. The only place where you can get married on the first date.

Real Life ;P (yup ur life ;))

Bonetown

Call of duty series XD

Go outside.

Normal simulator- ETS 2 so far,
Other: Arma 3 mby

RaceRoom Racing Experience.
And it's free.

Farming simulator

Dwarf Fortress.

LKW Traktor Bagger Simulator.

Or Cart Life.

I don't know if it "counts" as a simulator, but I really like Universe Sandbox.

GTA V

Surgeon Simulator xD
ARMA OA - War Simulator... Name not included
Flight Simulator - Not on Steam

Psychonauts

Girlfriend simulator

its called your right hand!

Depends what kind of simulation you expect. As for flying and racing:

Well there is DCS for modern combat flying machines, yet old Rise of Flight gives more joy for a casual (yet focused on strict simulation) player. And IL2 Sturmovik:Cliffs of Dover is quite interesting, but unfortunately broken. Hard.

GT Legends is old but damn good game that provides a lot of joy. That company released quite an amount of titles before and after that game, and every is worth checking out, especially GTR2 and RaceOn. Yet more accurate are some mods for rFactor (which while raw, is only a base engine with few playable examples), and ever moar accurate is iRacing based on same engine, but heavily modified but with monthly payment and ouf-of-steam.

I just played the demo for Euro Truck Sim 2, think its 50% off, it was chill as heck. Go check it out!

Street Cleaning Simulator

Under the ocean 2.49 right now too!

Schwebebahn Simulator 2013