The best tablet computer depends on your needs...
If Windows OS compatibility and a digitizer pen are important to you, consider the ASUS Eee Slate or HP Slate 500. The biggest differences between those are that the ASUS has a better display (12.1 inch AFFS 1280x800) than the HP (8.9 inch TFT 1024x600) but is larger, heavier and more expensive. Both the Eee Slate and the HP Slate 500 are expensive, however: Eee Slate is ~$1,200; HP Slate 500, ~$770. A more economical Windows 7 tablet is the Acer Tab W500, which runs about $500 and includes a keyboard and dock.
At the low end, $249 can get you a Nook Color, which has an ereader orientation but is really a budget, 7-inch, camera-less, tablet running Android 2.2. Better than the color Nook is the new Kindle Fire ($199). If you can do without cameras, SD slots and 3G, the Kindle Fire offers great web browsing with its new (cloud-accelerated) Silk browser.
If high-end video recording is important to you, consider the LG G-Slate or BlackBerry Playbook, both of which can record at 1080p.
For overall value, the ASUS Eee Transformer (TF101) has surprisingly good specs for under $400: Android 3.1, 10.1 inch 1280x800 IPS (like iPad, but bigger and higher resolution) display, up to 9 hours of battery life (16 hourass if optional dock added), optional ability to dock with keyboard, 1 GHz Tegra 250 ARM CPU with 1 GB RAM, good array of ports (mini HDMI, SD, micro SD), preloaded Polaris Office 3.0 for editing documents (.doc), spreadsheets (.xls) and presentation (.ppt) files. Note, though, no 3G (however people report success in tethering it to smart phones).
The iPad (0.34 inches) and Galaxy Tab 10.1 (0.336 inches) are the thinnest. The Galaxy Tab 10.1 (1.25 lbs) is the lightest of the big-screen (greater than 9 inch) tablets.
iPad 2 Advantages:
IPads have a higher degree of fit-and-finish than the competition. Like iPhones vs other smart phones, iPods vs other MP3 players, or Macs vs PC's, iPads are widely regarded as being more refined and polished than the competition. iPad also has a large and refined App Store (although Android Market is closing the gap).
iPad 2 Disadvantages:
1. No Flash.
2. No SD expansion; no HDMI port; no USB port.
3. ASUS Eee Transformer has a better display (10.1 inch, 1280x800, IPS vs 9.7 inch, 1024x768, IPS) for less money.
4. PlayBook and G-Slate have better rear camera video recording (1080p vs 720p).
5. PlayBook has much better front camera video recording (1080p vs VGA).
6. Almost all tablets have better camera resolution than iPad's lousy 0.7 MP.
Motorola Xoom is heavier and thicker than the iPad 2 and Galaxy Tab 10.1. The Xoom's advantage is that it has HDMI out and micro USB ports. It also has a 5MP rear camera compared to Tab's 3MP. The Xoom has a front webcam and a 720p rear video camera; the Tab has only the 720p rear video camera.
Finally, if cellular service is important, you may want to consider which tablet your favorite carrier supports:
None: Eee Slate, Slate 500, TouchPad, Thrive, Eee Pad TF101, Iconia Tab A500, Archos 101, Archos 70, and NOOK Color
AT&T: iPad 2 and Galaxy Tab
Sprint: PlayBook and Galaxy Tab
T-Mobile: G-Slate, Streak 7, and Galaxy Tab
Verizon: Galaxy Tab 10.1, iPad 2, Xoom, and Galaxy Tab