Hi RAMAKANTA,
Welcome to the forum and to the Arduino community at large!
There are MANY options to select from, so it really depends on hat you intend to use it for. Most likely, you just want to experiment with it and do some fun things.
I have an UNO (R3) and am quite happy with it. Most of the shields are designed for it, so I think it is the most versatile.
Pat.