I am planning to buy a bike to carry me to my work. What do you think is best in traffic? The single speed or multi gear?

What type of traffic/environment/distance? I haven't ridden a single speed extensively, but I can't imagine using one anywhere except a flat slow speed urban environment. I live in a hilly and mountainous area and my road bike has a small third "granny gear" front sprocket for steep hills as well as a larger large sprocket which helps pedaling down hills. Selecting gearing that allows for maintaining a comfortable pedal cadence is less tiring than pumping up inclines in too high a gear.

Buy a single speed bike that thieves are less likely to steal.

I would suggest a multi-speed. The reason is that starting from a stop on a single speed may cause some wobbling. Which you don't want in traffic. If you shift down when you stop it will be easier to get going again.

Also, the terrain you will be riding through may make a difference.

One of the many inventions with which Leonardo da Vinci is credited is the Continuously Variable Transmission (CVT). They are now being adapted to bicycles by a company in Texas. You might want to check them out, if you have the ability to securely store the bike.