Which is the best aura for the Paladin, I have only used thorns. Also which act 2 mercenary is the best one?
- For a solo player that is.
Early on, Holy Fire.
End game for melee, Fanaticism.
For Hammerdin, Concentration.
I prefer Blessed Aim for Merc so I can add more to Vitality than overdoing Dex.
Well, both questions are going to result in several different answers, because it’s all situational on what class/build you’re playing, your playstyle, what gear you have, PvP/PvM, etc…
Concentration aura is good for Hammerdins.
Fanaticism is good for a zealdin.
Vigor is good if you want to run like the Flash. (goes with Hammerdin)
A2 merc:
Might aura is good for extra physical damage
Holy Freeze aura is good for crowd control
Defiance is good for extra defense
Prayer provides a constant source of health regen for everyone in range.
Blessed aim gives you a huge boost to attack rating.
Thorns of course reflects damage.
Running with a cold Blizzard sorc, I usually go for prayer A2 merc just to keep my and the mercs health up.
What type of Paladin are you wanting to make?
Right now I really do not know. I like to run solo so I do not need to keep up with others. I only played one Paladin back in the day and I am sure I did not know what I was doing. I am looking for a good basic build I am not a pro. My first run will mostly be looking at all the updated visuals.
Thanks for the feedback so fare any additional pointers would be welcome.
If you got 25 minutes to spare, you might wanna take a look at this comprehensive video from MrLlamaSC.
He gives a general overview on the paladin and its skills and suggests a playstyle which comments on most of the auras.
Another general guide is this one:
If you wanna learn more about the individual playstyles then here is another
Zealot guide
As well as a Hammadin guide
There are of course other builds, such as smiter, tesladin and so on. But i think zealot and hammadin should be the most solid ones to start out with.
I even found a thorns guide, it seems that the creator is able to run hell baal with it, which is admirable, but i think he is using very high end gear to make it work. (e.g. a runeword with Jah + Zod rune in his weapon)
There are not many thorns pala guides around, which is an indication that they are not the strongest/easiest way to go.
Edit: after checking it seems to be more of a smiter build with left-over points placed into thorns for fun.
There are a lot of good straightforward builds you could use. If your looking for a Melee build I would suggest a smiter or a zealer both of which benefit greatly from fanaticism. A lot of good builds you can look at online as blash has listed. Have fun!
Depending on your build, also holy freeze can be good later on. I think MRLlamaSC once had a nice holy freeze build for endgame.
Other than that, also vigor can make a huge difference, if you make boss runs and have no enigma yet. If you reach Pindle faster, that means, you can do more runs and have more drops in the same time.
Also the „dead monsters Rest In Peace“-aura (don’t know the English name) can be helpful for nihlathak or baal‘s second wave.
There is also an Auradin build, which is based on offensive elemental aurae (Holy Fire, Holy Freeze, Holy Shock). It is an advanced build, however, and it depends on expensive gear providing offensive aurae (the character invests skill points into their synergies instead).
Here is a sample guide:
https://www.diabloii.net/forums/threads/clervs-pvm-auradin-guide.550647/
Conviction is kinda nice
Definitely not thorns 
It depends a lot on your build. Best is probably concentration as its improve Hammer damage and also boost dmg of your mercenary. But different builds use different auras. Hammerdin is best for solo play to finish whole game tho.
Start with combat merc on normal for healing aura, you can switch to one with holy freeze or might on normal. I would prefer holy freeze merc if you are playing melee paladin who attacks with weapon. It really helps. Its also good for hammerdin but i usualy prefered the merc from normal with defiance to keep my def really hog so monsters do not hit me.
If you would have additional questions feel free to join our discord and ask or PM me there or someone else will answer it if you ask in public channels.