Any good suggestions for Basys2 optional oscillator 50 MHz or 100 MHz?
Basys 2 already has integrated LTC6905 oscillator (IC5) that can be controlled with a jumper (JP4). You can easily obtain 100MHz, 50MHz or 25MHz just by soldering the jumper, so I would just recommend doing this. This silicon oscillator is connected to input B8 of FPGA. For most applications it will be more flexible to just use the integrated oscillator. 

In a few special situations you might want to install the optional second oscillator. For example if the frequency you need is not easily obtainable by dividing the one from integrated oscillator, or when you need a signal of greater stability (LTC6905 is not very stable). Note that the second oscillator is connected to a separate input M6, so you can use both of them at the same time. 

The example part number for 50MHz oscillator would be ABRACON ACHL-50.000MHZ-EK.

Of course the question what exact frequency you need depends on your application. You never explained what do you intend to do with your Basys 2 board.