Which Ubuntu flavor should i use?

You got few choices when it comes to ubuntu distributions, there is the original ubuntu, the light weight lubuntu, the xface xubuntu, the kde kubuntu, ubuntu mate, ubuntu kylin, etc. The big question is which one you should use? which one is better?

To answer the questions you need to ask your self, what kind of person are you? do you like gnome desktop? xface? lxde? or you prefer kde desktop? what kind of computer you have? old computer or brand new with the latest and greatest technology? Answering these questions will help you choose which distro/flavor you should pick.

For Old Hardware
If what you have is an old hardware, i mean like really old one, like pentium 4 or below, the best choice is to go with lubuntu, the lightweight ubuntu, you can also try xubuntu which technically also compatible with old hardware, another option is ubuntu mate, also requires minimum hardware specs.

If possible try to use the 64 bit version if the hardware support, because 64 bit machine have better support in terms of software, there many popular software that abandoned 32 bit system and just focus on 64 bit, so if you use 64 bit operating system you got better support for newer software.

For Newer and Powerful Hardware
If hardware is not a problem for you, then you can choose all of the available options, it comes down to what you like, do you like the unity on ubuntu? or prefer windows-style of lubuntu? or you like KDE desktop environment?

I strongly suggest you try for yourself, which desktop environment do you love most? unity, gnome, kde, lxde/lxqt, xface or mate desktop environment?

Ubuntu 
The standard ubuntu is using unity desktop, it is resource hungry, requires good hardware specs.

Kubuntu
Kubuntu uses KDE desktop environment, kubuntu also resource hungry, but maybe less than ubuntu.

Ubuntu GNOME
Ubuntu GNOME also resource hungry but uses pure GNOME desktop environment with no unity

Xubuntu
Xubuntu uses XFACE desktop environment, it is not resource hungry, requires minimum hardware specs.

Lubuntu
Lubuntu uses LXDE (soon will be replaced by LXQT) desktop environment, it is the lightest ubuntu (lighter than xubuntu) flavor you can get, if your computer can't run lubuntu then other flavors might not run at all.

Ubuntu Mate
Ubuntu mate also considered as light, it requires minimum hardware but not as light as lubuntu, ubuntu mate uses mate desktop environment which based on GNOME version 2, ubuntu mate kind a look like old ubuntu in my opinion.

Ubuntu Kylin
Ubuntu kylin is special distro design for chinese people, if you are a chinese or speak chinese, you might want to consider ubuntu kylin.

I personally like xubuntu and lubuntu, xubuntu performs very well on my laptop nice and smooth, while lubuntu is definitely best for very old hardware. So those two are my favorite when it comes to ubuntu flavors.