high frequency op amp for Arduino frequency counter
Try an op-amp in comparator configuration - nothing in the feedback loop, ground the non-inverting input (or use an offset as needed) and your signal to the inverting input. No feedback. If you use a chip designed as a comparator, like LM319, it won't have internal compensation and is much faster than an equivalent op-amp. 

LM319 has full scale response of 80nS (can stretch out to 180 with over-voltage input) which fits your frequency needs. Use capacitor coupled input for zero-crossing detector, or add about half your full scale input to the + pin. LM319 works with single +5 supply but so will a lot of others. Note the old standby LM311 you will see in a lot of examples is too slow.