Will point you in the right direction, but use with others
TripAdvisor is the behemoth of crowdsourced travel guides: their stickers are everywhere when traveling. They do have the most reviews and pictures for the most cities, so it’s hard to go wrong, but it's also hard to go very right. Their reviews are very tourist focused and generally middling quality so in order to find real gems it takes a lot of digging. The design of their apps are lacking too: interactions like filtering, sorting, mapping, etc are all a step below apps like Foursquare and Yelp. The main bright spot is you can download a city's reviews for usage offline, a rarity among these apps.
We recommend using TripAdvisor as a starting point but to supplement with other city guides to get a better balance between tourist-y must dos and off the beaten path or local gems.
Great in the US and some of Europe
Yelp is great in areas Yelp is used, which is mainly the US but increasingly parts of Europe and South America. Compared to TripAdvisor, reviews on Yelp are generally higher quality and more from locals, so you’ll see a slight bent towards restaurants, cafes, bars, and businesses than tourist attractions. The design of their apps and website are also a strong suit.
International content, great design, and useful personalization
Foursquare started off as a trendy check in app (remember that?), using that to grow into the full blown city guide it is today. It consistently has content all across the world, but there's generally not a huge density of it like you see on TA or Yelp and instead of reviews they use concise tips which can be good or bad depending on how in depth you want to go. Their design is the best in this category: sleek and easy to use with fun details sprinkled throughout 👌🏼.
A neat feature, especially when in a new city, is Foursquare's use artificial intelligence to provide personalized recommendations based on where you've been before. Love burritos? They'll automatically notify you with a list of nearby Mexican restaurants with bomb burritos.
Don't just check me out, check me in
Foursquare broke off their "check in" feature into Swarm, a separate app that's similarly well designed and fun to use. It's a great travel companion to log and remember where you've been.
Enough content to be useful and conveniently integrated across all Google apps
Google's local reviews can conveniently be accessed through any of their services: when searching on Google ('things to do in [city]'), Google Maps, their new trip planning app Google Trips, and others. There's going to be reviews for nearly everywhere you go, but it's rarely the best or most plentiful. The one area they are consistently great about is having 360 panaromic style photos for major attractions, which can be useful when planning.
Considering that nearly everyone probably uses Google in some way everyday already, it's an easy and quick place to look.
A more personalized, fun approach to local recommendations
Trip.com (damn, what a domain name) uses time of day, weather, the type of traveler you are, and more to provide timely and personalized recommendations. It's a different approach than other crowdsourced city guides that can work well if a city on Trip.com has enough reviews (which it does for most). As for the app itself, it's cute and designed with "gamification" techniques like badges, points, leaderboards, etc similar to Foursquare/Swarm to incentivize people to write reviews.