Welcome to RFF, Subhash! Having begun medium format long ago with a Pentax 6x7, the 645 format rangefinders all seem light and compact in comparison! I have four; they are all different and all have their strong points: GS645S (60mm f/4), GA645Wi (45mm f/4), and two Bronica RF645 (one with 100mm lens framelines, the other with 135mm). I think the Bronica is the most versatile, solid, Leica-like. The GS is very small and plastic-light with a great lens and a fairly traditional user interface with controls and settings around the lens barrel. And a noise-maker built into the shutter release. While the others take 16 shots on a 120 roll, this one gets 15. I like it, but find I don't use it as much.
The GA is entirely automated, like a giant point'n'shoot, complete with motor wind, autofocus, popup flash, etc., but it too has a great lens and excellent build quality. It has buzzy little motor noises to extend/retract the lens and wind film.
I like the GA's 45mm lens (it comes as a 60mm version too, and a slower 55-90 zoom) and dedicated viewfinder with frame-size correction as well as parallax compensation. The AF is both a strength and a weakness. But I think Iike the Bronica best, for its crisp bright viewfinder and RF spot, manual and AE modes, quiet manual film wind and quiet shutter.
Each of these has something for a user to prefer, though. And they all have that extra something in the film results common with medium format...