Depends what you need snd what you want to do. Its like saying, what the best boat or what s the best car... I ve used both Marine Traffic and Boat Beacon. MT has much better coverage But BB allows you to broadcast position and data over the internet making it useful for friends and family to follow you
A lot depends on the area, around here I've found Vesselfinder and Marinetraffic use different shore stations and each has pretty different coverage areas and gaps.
Are you looking for an AIS app that is getting data from the internet? Or for use onboard with your own AIS transponder or receiver?