I'd rank based on hair-pin potential, complimentarity and potential for self annealing. Finally use annealing temp to decide the final 20. 

You may find this a good tool: http://www.basic.northwestern.edu/biotools/oligocalc.html

regards

If you are going to use them on a very large template (like total genomic DNA) you may also want to check for specificity.

Dear,

There are several parameters you can check with them:

1. check flexibility region ( repetitive nucleotides)  like G-C rich region and mask it by RepeatMasker (http://www.repeatmasker.org/cgi-bin/WEBRepeatMasker)

2. You have to check this primers have secondary structure to prevent primer-dimer by http://yellow.nist.gov:8444/dnaAnalysis/primerToolsPage.do

3. calculate Tm, from https://www.neb.com/tools-and-resources/interactive-tools/tm-calculator  or manually by this formula: Tm = 4(G + C) + 2(A + T)°C . The best value should be in between 52-58°C.

4. Calculate annealing temperature by this formula :  Tm-5

Kindly write to me for any further discussion

Thank you