Reception all depends on what market / area you are in.
For heavily populated urban centers with a lot of high-rise buildings, you can' t beat CDMA technology (Verizon and Sprint PCS), and even within CDMA carriers, Verizon is a lot better because they use a lower 800 MHZ band vs Sprint's 1900 MHz band.
So, if you are in San Fran or New York City, Verizon is the best carrier. Nextel is next on the list because they also use 800 MHz frequency. (Sprint uses 1900 MHz frequency because Verizon and Nextel has bought the rights to 800 Mhz band before Sprint did). In these areas, Verizon can receive calls inside elevators, in the subways, etc where you would not even get signals from GSM carriers. This is a documented fact and it is based on certain technology limits that relate to the frequency band your carrier is using.
Now, if you are in a more sub-urban area, then GSM carriers (T-Mobile, Cingular, ATT) can be just as good as CDMA carriers. At this point, it comes down to which carriers have more towers in your area to provide coverage. For example, if ATT has more towers in yoru area, they will be better than Nextel, etc or vice versa.
So the answer is, it all depends on where you live. You have to just ask around in your area.