What would be the best flight simulator for a beginner to buy. and why?

A used one. Check out THEBAY and RCU marketplace.

Anyone of the popular ones would be a safe investment.

They're popular because they're acceptably decent as a sim to the majority of their users. And being popular, they've probably got their own forums and support somewhere on the internet. You can swing into those forums and get a fair idea of the value of them to some of their customers. For example, G3 has a large following and has a number of special interest forums on their website. They probably have the largest number of copies sold, but don't expect their forums to be very busy however. A lot of sim users don't have much to say about the sim. After all, they're more interested in RC models, and what's to say about a sim compared to what's to say about RC models.

Check G3's forums out..... One is exclusively for sharing user made models for the sim.....


http://www.knifeedge.com

there are a few free ones floating about on the net, if you are just wanting to get familiar with the charecteristics of flight then get a checp copy of Microsoft Flight Simulator 98 or another VERY good one is a sim called "PREflight" its very good, lets you use your RC transmitter as a controller and has helis, planes and you can go to their website and get downloads (such as snoopy riding a dog-house, a chinook and a concorde)

but go with a well-known and approved brand

Google FMS and have a look. This is a safe download and free. Look at LINKS on the site for interface cords.

Has a beginner myself and I tried different sim, I can tell you my favorite is the Realflight G3. It very realistic.

Be carefull to check system requirements before buying any sim. I would hate to spend $200 to find out my computer can't handle it.
That is why I went with Real Flight G2 (can be found used on Ebay). It was less work for the computer and it runs well on my laptop.

AeroFlyProDeluxe, free downloads, frequent new planes and sceneries, very realistic, lot less lockups, hangups, downtime.

G-3 rocks but like missileman pointed out sometimes a comp. upgrade is needed for it to run properly I had to upgrade my vid. card to get it running now I dont know what I would do for a quik flight fix at 11:30 at night.
there is also a huge section on RCU about the sim's read away.

We did a side-by-side taste-test comparison of G3 and Aerofly Pro Deluxe at one of our club meetings. Everybody who has G3 at home liked G3, and everybody who had AFP at home liked AFP. Interesting results.