As a few others have said, Druid. Especially Night Elf because of Shadowmeld.
As a warlock, you have to really try to die playing solo.
It really depends on what you want to do. For questing, things like Rogue’s stealth and Druid’s flight form can make a huge difference. PVP would be whole other topic. If you like to queue for dungeons, a class like Paladin, that can do heals/dps/tank roles whenever, gets you in quick. Doing older raids for transmog and mount hunting is quicker with more mobile options like a lock, dh, or warrior. If you are all over the place, a mage’s teleports are handy.
If you get good enough with any class or spec, you can solo most things comfortably, so my ultimate advice is to pick something that you really enjoy playing and become an expert at it.
Hunter, Warlock, or any tank if you want survivability.
Main difference is that Hunter will kill stuff faster than the others.
Druid if you want utility as you can switch specs for any role.
Rogue is fun because, well… rogue. 
(and yes this is a solo character)
My thought would be to make class trials and go with whatever one suits you best.
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Solo player, BM hunter or Guardian druid.
Solo content is trivially easy and doesn’t really exist. Play whatever is fun.
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Play a rogue. Best solo class imo. Can sneak around and pop big cds and defensives for tough mobs.
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Arcane mage. I’m joking. Warlocks and hunters can solo anything outside except for current xpac world bosses.
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Oo the best to paly is the SHaman D its so fun
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The question was “solo”. An elemental shaman is not even remotely ideal for strict solo play.
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Agreed. This one. I’ve never played a Pally but as a solo the other three are fun as hell!
You have previously played so you’re fairly familiar with the classes I assume. I would go with whatever class/spec that you really enjoy playing. I prefer Lock or Druid for any content. Lock because of lore and imo locks are total badasses. Having a pet tank and a good tool kit are a plus too. Druids are great because you can heal yourself, tank for yourself or dps like a beast (pun intended). Having travel forms is really nice too, especially if you are a gatherer.
TL:DR Lock or Druid because of lore and a great tool kit.
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For pure survivability than blood dk can’t really be beat. You can solo world bosses as long as you have the time.
Pet classes of course are usually pretty strong for soloing. Healers are probably the worst for soloing content, unless by solo you mean always in the random dungeon queue.
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Any class that can tank.
Just stick in your dps spec for most content as tanking will slow you down.
And when something hard comes up, swap spec.
I would avoid druid unless you really like the 4 races that can play it. And also love the idea of being shapeshifted 99% of your time.
Personally i cant stand it.
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DH. So tanky in either spec, good at mowing down a lot of mobs. Lots of self healing.
Anything that can heal probs
Paladin, Prot/Fury warrior, Survival Hunter are what comes to my mind.
If we’re talking fun to play I’d put Ret Paladin pretty far down the list. Some of the specs can lead to low button throughput which feels janky at best.
Most fun is probably Rogue. All three specs feel good to play.
Next up… uhhh… Shadow Priest? Not easy, not hard. More kind of annoying but rewarding in the same weird way.
WW Monk is fun but it’s got a learning curve to it. Same with Druid anything.
Tank wise is probably Prot Warrior.
Healer wise is uh… Shaman? Dunno I haven’t tried one. Paladin’s been the most fun but they’re not exactly great healers right now.
Hunter’s that sort of class you play if you know you want to be one.
Warlock feels iffy. Affliction used to be fun but they neutered it.
Mage? Same as Hunter.
Death Knights are GCD locked and it’s annoying.
Evokers don’t exist.
I would say without a doubt, rogue.
With stealth and vanish, you can practically pick and choose your battles.
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Druid.
Ranged. Melee. Stealth. Healing.
You can spec into tons of crowd control.
I mostly play solo with this Balance Druid. Moonfire + Sunfire + Starfire to take down giant groups of enemies.
Typhoon, Mighty Bash, Entangling Roots, Ursol’s Vortex to keep everything away from me. Treants to grab aggro temporarily if I need a moment to heal, and I specced into the instant self heal.
It feels amazing to play.
I also love Affliction Warlock. I keep the Voidwalker out for world content as my tank, and throw Seeds of Corruption at masses of enemies and watch them all explode. Very satisfying.