BlackBerry is a very stable and powerful smartphone. The latest versions of BlackBerry operating system provide a large range of multimedia, third party application, game support besides its traditional instant push email. From the requirement you have listed a BlackBerry is certainly a good choice.
To benefit from the improved features it is advisable to choose a reasonably new BlackBerry models. Such as the BlackBerry Bold 9000 (GSM), Curve 8900 (GSM), Tour 9630 (CDMA), Curve 8520 (GSM), Curve 8530 (CDMA). The first three are from the high-end range, while the last two are from the medium range. Compared with older models, they have better browser, higher screen resolution and HTML email support.
RIM offers far more options on GSM networks than CDMA networks. And they news phones usually come to GSM networks before, if ever, being released on CDMA networks. Therefore, you are more like to get the latest phones from AT&T or T-Mobile than from Verizon or Sprint.
Your requirement includes: Internet, Wi-Fi, E-mail, Calendar, GPS.
All BlackBerry phones support Internet, E-mail and Calendar. And the high resolution screens (480x320 to 480X360) featured on the newer BlackBerrys are very good at displaying web pages. If you plan to use the phone to watch video streaming on the move a lot 3G (either HSDPA or EVDO) support might be useful. Some models for example the 9000, 8900, 8520, 8530 support Wi-Fi. All the newer models listed above supports HTML email, while older models only display email in plain text form. All BlackBerry models have calendar and you can use Google Sync from Google to sync your phone's contacts and calendar with that from you Gmail account. Only a limited, but growing number of BlackBerrys support GPS. Within the 5 models above 9000, 8900, 9630 have GPS, while 8520 and 8530 do not.
In conclusion, you should possibly consider a Bold 9000 or Curve 8900 from a GSM provider (AT&T or T-Mobile). These two models are quite similar, except 9000 has 3G support, Wi-Fi, GPS, a 480x320 screen, leather back and mini-USB charging port, while 8900 has only 2G support, Wi-Fi, GPS, a 480x360 screen, newer micro-USB charging port, smaller form factor and a lower price.
Bear in mind that the successor to the Bold 9000, the Onyx 9020 is expected to be released between September and November. If you prefer a latest model, do not mind waiting for a couple of more months and money is not an issue (most networks have lowered the price for the Bold 9000 and Curve 8900 right now before the release of newer models), you can consider getting the Onyx 9020, which is based on the Bold 9000, but with the Curve 8900's 480x360 screen, mini-USB charging port, smaller form factor and instead of the traditional trackball, it will feature an optical touch pad.