You can use total station for finding elevations which is accurate.

Conventional land surveying methods/ total station survey can give you highly accurate DEM. The second option is LiDAR derived DEMs.

LIDAR point clod data or fine scale GPS point data are accurate.

A DGPS survey would be another effective method to prepare a high accuracy DEM.

Cartosat DEM

If u want use open source data, SRTM is better or otherwise u need to depend on high cost data, again it based on ur fund availability. You can also try by extracting elevation from google terrain, which is better than SRTM data.

Try ASTER DEM as well...