There really isn't a "best" Stamp. They all have particular advantages. In general, I'd suggest using the BS2p series (p/pe/px) unless there's a cost or availability issue in favor of the "plain" BS2. This is mostly because the BS2p series has a lot of additional statements that can make I/O simpler. The parallel LCDs become almost trivial to use. I2C devices become easily accessible. The STORE statement makes it much easier to use extra EEPROM banks for data storage. I haven't found the POLLxxxx statements too useful, but I can imagine some specialized applications where they might make a difference. The BS2px has the speed to replace the BS2sx and has the pin configuration options (pullups, Schmitt trigger inputs) that can help a lot in connecting the Stamp to contacts and pushbuttons.
If I had to buy a new Stamp and could choose any of them, I'd get a BS2px because of the speed and extra features.
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If I had to buy a new Stamp and could choose any of them, I'd get a BS2px because of the speed and extra features.