It depends on what features you are willing to trade off. The Xoom has excellent build quality, mini HDMI, SD Card slot (read only at the moment), and mini USB. It also gets software updates first because it is running stock android. However, its screen is inferior to both the Transformer and Galaxy Tab 10.1. With the recent price drop, I think the Xoom is now the cheapest of the bunch.
The Transformer has an excellent IPS screen, mini USB, mini HDMI, SD Card slot (read and write), and Asus seems to be on the ball with its software updates. The Transformer also connects to the optional keyboard that turns it into a mini-netbook.
The Galaxy Tab 10.1 is the most "ipad-like" of the bunch. Its screen blows all of the other tablets out of the water. However, there is no mini USB, mini HDMI, or SD card slot. You would have to buy Samsung's proprietary adapter to hook it up to your computer. Also, you have to deal Samsung's TouchWiz interface.
The Toshiba Thrive has a Xoom-like screen, full USB, full HDMI, full-size SD Card slot, removable battery, but reviews have noted that it lags more than the other tablets.
HTC just came out with its 10.1 inch Jetstream through AT&T, but it is the most expensive of the bunch. There is also no WiFi only model at this point.