Please suggest the best tools for OSM user activity monitor.
You can start with http://hdyc.neis-one.org/?username
For seeing who did what near you yo can use http://simon04.dev.openstreetmap.org/whodidit/ or one of the other “whodidit” instances out there (see the OSM wiki). You can get an RSS feed too.
For small areas, a view of the changes in the last week http://tyrasd.github.io/latest-changes/