Which is the best module for notifications and subscriptions for D7 which is actively maintained and has a stable release ? .. Is there any which exist ? What are the possible alternatives that one can use to implement the subscription and notifications via email to a group of subscribed users ?
Without knowing more specifics about your use case, it's not clear which one is "best." Notifications & Subscriptions modules differ in important respects, though the former is actually a fork of the latter. Subscriptions, being the older one, is both more straight-forward as to what it does, and also a bit clunkier on the code side (although it did get a major rewrite by chx for the 2.x branch.) Notifications was a rewrite of Subscriptions intended to make it much more extendable but this is not to say Subscriptions isn't itself extensible; they're both modular frameworks.
If all you need is a way for users to subscribe to nodes you may find Subscriptions does everything you need. If however you need to support more integration points, Notifications may be the better match. I think it has a lot more modules in Contrib that already extend it, and is being used on about 3x as many sites as Subscriptions (not sure what the breakdown is for just D7 sites.)