Lonely Planet Guidesworks offline
Beautifully designed app with Lonely Planet's signature quality
Lonely Planet, historically known for their dead tree books, has quietly reimagined itself for the mobile age with their new Guides app. It's a joy to use: a clean and straightforward interface, powerful filtering, offline ability, convenient bookmarking, and beautiful pictures. Oh, the content is also to the point and well vetted with all the detailed information you'd need like the typical cost for lunch or dinner. Although it started off sparse they now offer over 100+ cities. We're big fans.
The New York Times: 36 Hours
Succinct and practical guides with balanced recommendations
The basis of this series by The New York Times is what to do if you only have 36 hours to get to know a city. They're stylishly written and very practical. The best part is they don't just recommend the tourist traps but also places that are more local or off the beaten path so you get a better idea of what the city is actually like. The more recent ones also come with helpful annotated maps to get your bearings. These should be your go-to if you're pressed for time.
Prefer books? They sell region specific compilations like this one for Europe
The Wikipedia of travel city guides
Although not technically "professional" by strict standards, Wikivoyage is high quality enough to be considered so. Wikivoyage is modeled after Wikipedia in that information is added and edited in a crowdsourced manner but publicly checked and held to higher standards than what you'd find on something like TripAdvisor. Each city is well researched making it similarly fun to fall down the "oh this looks interesting" kind of link clicking rabbit hole we all do on Wikipedia.
Anyone can use Wikivoyage's data since it's publicly licensed, so for iOS we recommend Modern Atlas. It presents Wikivoyage's data much more eloquently.
For finding cool and off the beaten path gems
Atlas Obscura specializes in places you don't often find in typical guides, like an abandoned subway station or an island full of cats, making it great if you've already hit all the tourist must sees or you fancy yourself a weirdo (raises hand). The writing, visuals, and design are all well done and even integrate cool community features like saving places to a "want to visit" list.
Prefer books? See their best seller An Explorer's Guide to the World's Hidden Wonders