Hi all,
"go as high as you can."
OK, height is might but like the man said there comes the point of diminishing returns. Where that point is becomes a wild card without careful and complex calculations based on system gain (must include HAAT calcs, not the antenna gain alone) vs. transmission line loss vs. frequency. In other words a professional site survey is in order which leaves those outside the RF communications industry with a best guess situation.
A best case scenario would be perhaps sitting at the base of a 100' tower with a high gain antenna and Times Microwave LMR-800 or hard line but we should be so fortunate. Using that as a template you can take a well informed guess that with a good antenna with good coax mounted as high as practical you'll come out on top. Then being on top of the highest mountain in the area is better. If you live at the bottom of a hole you're SOL no matter what you do, thankfully we seldom encounter the worst case scenario.
Some install a good antenna up high and try tio cheap it out in the coax department, a real shot in the foot. Don't be one of those, use your noodle and THINK before you reach for the sky. THEN reach for the sky, this is a holdup! (;->)