To make matters worse, the phantom power on the MobilePre is underpowered, so a lot of condensers sound like crap with it, too.
My advice would be an AT2020. It's an electret condenser, so it shouldn't be as picky about how much voltage is available (since the element is probably driven at three volts or something insanely low). It's a good solid mic that has pretty flat frequency response, so what you hear is pretty much what you get, which when you're starting out is pretty important, IMHO. You can experiment with mics with more character later. The AT2020 is also a cheap mic that you might actually keep using for some stuff later, unlike most of the other crap in the sub-$100 range.
You might also consider some of the MXL condensers, but I'd wait until somebody who actually owns a MobilePre says that it works decently. That's quite possibly the worst preamp you could buy. If it isn't too late to send it back, send it back. Otherwise, I'd start by adding an
Art Phantom II to the thing to get true 48v phantom power.