A long time ago paper maps were the only good option to find your way up the hills.
Still I like to use them when leading a big group, to show the spots where we are heading or to teach the basics of navigation with the compass, etc. But in my day to day, I rely way more on my phone and GPS watch. Actually, I bought my very first smartphone when I saw the potential that they had for my outings.
I miss original Everytrail, which combined in just one app cached topographic maps, GPS recording and photo geolocation where you could easily add notes all around, embed maps and play pics. But it was bought by TripAdvisor long ago, who neglected for years and recently was acquired by Alltrails, but it is not the same… In any case, I have been using different apps. Main requirements: possibility to download different maps to check them offline & GPS/compass accuracy (although this is usually more related to the phone hardware itself, check your manufacturer). Additional features to consider: option to save waypoints with pics, record your track, see GPX files, altitude, emergency messages, etc.
The app I use (I am an Android user) primarily is All-in-one offline maps. It is quite simple to handle and allows me to download, for later offline usage, Google maps and mainly Openstreetmaps. The Cycle/Hike maps actually which match the OpenCycleMap web, which I frequently mention on this site.
They provide tons of paths all around Hong Kong and the world, contour lines to see the elevation and slope, streams in blue, etc. If you are preparing an outing in a specific place you can “Select and save an area” with the zoom level most interesting for you: the more details you want, the heavier the download will be (wifi highly recommended). If you just check around it will keep in the (SD) memory the latest things shown too. In my case, as you can see above, I have around 250MB downloaded that suffice for 1/3 of HK full zoomed. In some remote areas where the vegetation is not so thick I like to download also the Google satellite images, as can provide me some guidance of possible exit routes. When going abroad I also tend to download quite some maps beforehand.
The most similar app for iPhone that I know so far is ViewRanger (also available on Adroid). It allows you to access to the Hike maps offline and has its own additional functionalities: record your tracks, you can share your location & track online, alert in case you are in danger, etc.
If you want to plan an outing consider using Trailwatch. Available both in iOS and Android and specific for Hong Kong hiking. It uses several maps including Opencyclemaps to provide you possible routes. Mark the starting point, end, even via points and the shortest route will be marked.
It might not be the optimal route (it doesn’t differentiate between main paths and more obscure ones where vegetation can be thick, for example), but the information provided can be really valuable. The distance & elevation profile can give you a sense of the difficulty. I use it frequently when looking for potential early exits in a not so familiar area, combined with the first app mentioned for the satellite images. Trailwatch can be used also to report trail and country park misuse, share your planned routes, etc.
Another similar application is Hiking Trail HK, only in Android as far as I know.
Some years ago and after quite some bangs my smartphone’s GPS started malfunctioning and I was doing longer and longer distances, which drained its battery when recording the path. I decided to buy a dedicated GPS watch: Garmin Fenix 2. On paper it was perfect for my adventures: 18h battery with continuous GPS track and over two days in saving mode; open water and pool swimming options, climbing, mountaineering and others; advanced metrics for running; a bit bulky but sturdy enough to survive my crazy stuff… The GPS chip and software “upgrades” in that model though were quite faulty and I needed to change the unit four times, till I got something “reasonably” accurate… Later iterations (Fenix 3 and now 5) seemed to have solved most of the problem and I have friends really happy with their Suunto.
Depending on the brand and model the functions and usability are different. Originally these GPS watches were targeted to runners and triathletes. Since some years ago major brands have units dedicated to multi-adventures: extra tough, with barometer/altimeter, possibility to show maps/waypoints and others. Examples: the mentioned Fenix series, Suunto’s Ambit or Traverse series, TomTom Adventurer, etc.
Pros: Way longer battery life than the phone. Super accessible (it’s on your wrist) in any conditions: while running, swimming, bad weather conditions… Related apps (Garmin connect, Movescount, Strava…) can be an incentive for some people to train more and allow you to easily check progress or find routes done before.
Cons: Expensive dedicated gadget if only for hiking (you might consider phone + an external battery instead). Screen resolution is limited and navigation not as simple as in a smartphone, therefore following a pre-downloaded route with them is possible, but more cumbersome.
If you are considering buying one (and you want to know a lot about possible options), I would highly advise you to go and read DC Rainmaker, best web I know so far.