Key points for DEM generation(By SNAP)

1-Baseline hould be between 150 and 300 (SLC pairs with a short temporal baseline)
2- the values in the coherence around 70% (above 0.4.)
3- short temporal separation

Please comment:

Have a look at this ESA document, it gives hints on how to get a good image pair. (chapter 1.3 Selecting images for InSAR DEM generation)