You can check out Aptoide, which contains ~90% of the stuff that's on google play, rendering it useless.

Also, you should probably read up on rooting, but you might wanna wait with that until you get more familiar with the platform. Without root access, you cannot delete the Youtube app for example (why? ask google...)

For driving, MapFactor is a pretty good free GPS navigator that works also offline (ie. you can download the maps to your device so that you don't necessarily need a net connection while navigating, albeit a net connection does help with finding more POIs). Being able to use it offline is a pretty big deal to me especially when using the navigator abroad.

You can either use the free OpenMaps maps with it, or buy TomTom maps. I've been using OpenMaps, they work passably even though they are not quite as detailed as many paid maps, but I like the idea behind it (map data that should be available for everyone, not controlled by a company like TomTom, Navtech or Google).

For playing video files, both VLC and MX Player seem good. I like the MX Player user interface a bit more, but then there are some video files that MX Player seems to have problems with, while VLC still plays them fine. I have both installed, but if you want to install only one, maybe VLC is a surer bet.

Go for fish fillets if you lije torture own brain:)

I've read about these kind of events various times inside reviews.
Basically any app is going to be ruined at some point. :(

Also, many of them request crazy permissions.
Luckly it seems that Android6 will re-add a security manager.

Anyway, do you have any opinion on these:
- 360 security
- NoRoot Firewall

And do you know a better alternative to:
- Calendar Snooze -> I need alarm and events sounds repetition!

I've used NoRoot Firewall and it does its job well.

Other apps that haven't been mentioned and I'd like to recommend are:

Sleep as Android
(Great alarm options and -optional- sleep tracking)

Cam Scanner
Neat tool to scan documents conveniently.

Also, what's wrong with Greenify? I use it without complaints.

Uh I'm not exactly sure, I just found articles like these

Oh, I get it. Yeah, memory boosting is kinda pointless. Personally, I find it useful to kill misbehaving apps or kill those naughty apps that run at startup without your consent. It's just faster than going to the settings menu and looking for those apps manually.

Oh, I forgot to mention ES File Explorer -it's really essential if you wish to see your file structure from your phone indeed! I probably forgot to mention it, because I use it way more on my tablet that on my phone.
Btw, ES File Explorer also includes an inbuilt image explorer, and if you click on an image file from within the app it proposes ES Image Explorer as one of the programs with which you could open it, so, assuming you get ES, you won't need QuickPic or anything of the sort...

FBReader for all your reading needs - smartphones aren't the best screen to read on, but since you'll always have it on you it's good to keep a few books on there.

Baen gives out free sci-fi and fantasy ebooks, as well as selling DRM-free ebooks directly so you can just download them and put them on your phone, no special apps needed. I really like Timothy Zahn and Sharon Lee/Steve Miller
Project Gutenberg has public domain classics.
MobileReads forum has a ton of information on where to get good legal ebooks.

Games:
LYNE
Threes
King of Dragon Pass
Buddy and Me

ScummVM lets you run adventure games on Android, so you can install that and copy over the data files. Adventure games work pretty well that way.

There have also been some PC games ported over like Shadowrun Returns and Knights of the Old Republic.

I use VLC Player for video and music, and ES File Explorer instead of the included file manager.

Damn, I'd really like an offline Calendar with events alerts repetition.
It was my favourite feature on my old phone.

Also, is it possible to make an offline backup clone image of the smartphone?

As in the apps, data, and the operating system itself, all backed up? Or just the apps and data? If just the apps and data, you're still looking at Titanium Backup if you have root. If it involves the operating system, I honestly dunno about that.

The only way I know to do something like that is to replace the stock boot-loader with TWRP (Team Win Recovery Project) boot-loader. TWRP will allow you to make backup images of the 5 different partitions of your phone (boot, system, data, cache, and EFS). I don't know how to put it on your phone since mine came with it already from Freedompop which all of their phones come rooted and have either TWRP or Clockwork Mod boot-loader pre-installed.

Why can't they simply let you have full control of your device from the start? -_-'

Because people, in general, are idiots and can't handle any greater responsibility than getting each of their socks on the right foot in the morning - often barely that - all the while refusing to accept that fact. Also, it's a bitch to listen to them complaining that they "did the same thing that's worked every other time, except this time the machine broke" and always putting the blame on me, so we technologically minded people try to get the devices to handle themselves and remove as much user error as physically possible.

Get a Jolla. Awesome clean GNU/Linux system. Root and a full-featured terminal with (most of) the common utilities is simply a checkbox away.

They could have just added a "full access" switch (or something) with a clear warning about losing license and support.
The truth is that they want to force their apps (like G+) and probably collect data.

Btw, Google Play is a mess: a ton of free apps are actually demos, adware or spyware.
The permissions request are often absurd and you have zero control over them.