If given the option to choose only one single mastery which one would you choose.
Your reasons are your own- comfortable to play with, survival skills, fun to play, easy cheesing, like the lore behind the mastery.
If I could only choose one it would be demo. It’s really fun, has good aoe, great damage, very tanky, lots of good support skills and can debuff the enemy making you even more tanky and them weaker. I think it would be the best solo choice for a pure class 
My first instinct with an ARPG is to go for a pure melee class, started with Diablo II since the demo only had Barbarian. But on this one the Class/Mastery used for promo images and loadscreens was the Occultist so that was my first choice and i must say it was a pleasant surprise. No way an Occultist can survive Ultimate without the right gear but the lore and the image in the Mastery Choosing page and also the fact that it reminds me of Necromancer (my personal favorite class) from DII makes it the class i would choose as a lone class
Shaman. Not because it is the best (soldier all the way if we’re going for most powerful pure mastery) but because it can be well rounded offense and defense while having more interesting skilling options than pure soldier.
Nightblade. I love to DW swords 
Demo for high damage reliable ranged combat.
My main is a Dual Wield Fire Striker Pyro. He is a beast and I love him
Can’t really be a Dual Wield Fire Striker without Demolitionist
Syn
None as I see no reason to limit my character to 1 mastery, when I am able to pick 2.
If you are asking which mastery is my Favorite its a tie between Arcanist and Demo (and when combined together they make me Happy in Pants)
While I really like Occultist as a support mastery that works well with almost anything, it is very lackluster as a single mastery. So I would vote for Demo in this case as it has a good mix of Offense and Defense as well as multiple options for builds.
Shaman for life.
Occultist, its my favourite mastery in all the game. I can survive with sigil of consumption and blood of Dreeg and i like the poison damage and the two summonable pets. I´m playing now with pure occultist with level 68 focused in pets and vitality/caos damage :D.
Used to be demo or nightblade but shamen has taken over, still find soldier boring to play!
Soldier, followed very closely by the Demolitionist.