I think the real key is to never get anything less than 100% agave. Contrary to popular belief, regular Jose Cuervo is not 100% agave (it likely has some sort of grain based liquor mixed in).
I agree with many that 1800 is a good value. That's usually my go to tequila. You can obviously pay more; however, if you want to keep it somewhere around $25 per bottle (maybe $30) 1800 is a good bet.