What is the crowd's opinion of Maverick?

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Line is an absolute requirement if you communicate with Thais. Other messengers are niche products, with far less use.

Google Maps is essential for traveling around. For Bangkok, it has public transit directions (BTS/MRT/buses). A separate offline maps app is handy in areas with no data coverage or neighboring countries (GMaps offline feature blocks download of some areas, most notably Bangkok). I used Maverick but there could be better ones.

Note that maps are for your own reference -- most Thais (e.g. taxi drivers) cannot read maps well, so pulling out a map won't help much in communicating.

Not an app, but I find this Bangkok bus list (use "find in page") useful for getting around.

Smartphone tip: if you're looking for a product (or a landmark), but can't explain it to a Thai (due to their poor English or your insufficient Thai), do a Google image search, then show the picture to them.

Google Translate is not great for Thai, but better than nothing. Simple words/phrases work better than complex sentences. You can pre-load Thai for offline use.

AirAsia (flights), Agoda / Booking.com (hotels) are services I use regularly. They all have apps, but I'm not sure of the quality since I always used them from a laptop.