The length of channel (L) is the ost important parameter. short channel devices hav good frequency functions but have small gain. Threshold voltage. probably you need som extra devices or layers in your design such as poly2 metal 3,... .
Selecting devices is depends on your circuit operation requirements.
For example :
Voltage range
Frequency range
Load current
Temperature range
Timing (Rise time, fall time etc. in case of digital usage)
Power disipation
etc.