It really all depends on ...

It really all depends on what your goals are and what your budget is. All models have strengths and weaknesses depending upon the governing equations. There are many models to choose from. There is not one "good" model. Please read the following links:

Report by the USGS  "Guidelines for evaluating groundwater flow models"

http://pubs.usgs.gov/sir/2004/5038/PDF/SIR20045038part2.pdf  

http://water.usgs.gov/nrp/proj.bib/hill.html

A very simple gw model is Processing Modflow by PMWIN. Version 5.3.1 is free. Lots of good tutorials go along with this software.

http://www.pmwin.net/pmwin5.htm