Best Indications of Low Signal/Bad Quality

Since starting this site in late July, DriveTester is up to about 375K of data points across all 3 carriers.  The plan is to eventually do monthly reports and come up with a rating system to to find out who was the best for that month.  I'm open for suggestions on how to best accomplish this.

I'm very critical of the data that is being collected.  For the reports that I run, I want to ensure that "Qual/RSRQ <= -20" indicates a consistently unusable signal. For the Level/RSRP, I'm still unsure on whether to use -115, -120, or -125 for the threshold to mark a carrier as unusable in that particular area.   Do I combine these 2 metrics (RSRQ and RSRQ) in one report or break them up? I could also do reports on when there is packet loss (ping-loss) or the Download speeds are less than 1Mbps.

I'm going to think about this some more and eventually come up with a rating system that is accurate.