Hey, @sajithmohan
I think it depends on what is important to your client and their specific needs.
Besides, Manychat charges “per contact”, but from the beginning, you have full access to Manychat PRO.
While Interakt and Wati focus only on Instagram and WhatsApp, ManyChat provides your client with seven different channels to work with.
Based on 10,000 contacts:
To have the full power in Interakt you need to pay: $63/month (2 channels)
To have the full power in Wati you need to pay: $155/month (2 channel)
To have the full power in Manychat you need to pay: $65/month (7 channels)
I hope this help your client choose the best tool based on their needs. 
@sajithmohan I also consider which companies are Meta Business Partners when making the decision.
You can check the directory here: https://www.facebook.com/business/partner-directory/search?solution_type=messaging&ref=fmp_about_solution_card
As you’ll see, Wati and Interakt are not.
For me, working with an approved business partner gives me some extra peace of mind - especially for clients investing heavily in growing their brand in Meta (FB/IG/WhatsApp).