World of Warcraft has quite a bit of endgame to experience. One of the options with a ton of replayability is Mythic+. Infinitely scaling dungeons with added affixes changing every week on 10 different dungeons. Through all this, not all dungeons are created equal, with some being far easier than others. So if you just want to get your loot as easy as possible or are trying to push your keys to high levels, this tier list should help.

Easy (these dungeons are the ones you want)

Atal'Dazar

With very easy bosses and fairly easy trash, Atal'Dazar is one of the best dungeons if you want an easy time. It also has many different paths you can take depending on the weekly affixes.

Siege of Boralus

Fairly easy bosses with very easy trash. Siege of Boralus is a good one for a quick M+. It is very linear and you will often take the exact same path every time.

Freehold

Freehold is very easy but might trip you up the first few times through. Definitely a good choice for pushing keys or for a quick run.

Medium (these are managable but might take a little more effort)

Underrot

Underrot's bosses are very easy once you know the mechanics but can be a little challenging otherwise. The trash can trip you up if you pull too much. Not too bad once you've run it a few times.

Waycrest Manor

Waycrest Manor is not too bad once you get the hang of it but can be very difficult the first few times through. Becomes much more difficult with affixes that limit movement such as Sanguine and Quaking.

Tol Dagor

Can be either very easy or very hard depending on how well you know the mechanics. Having a rogue or Jewelcrafter can help with lockpicking.

King's Rest

Some of the bosses will be very difficult if you are unprepared and the trash is scary when you pull too much or are unaware of their abilities. Not too bad if you understand the trash abilities.

The MOTHERLOAD!!

The bosses are not too hard but the trash is a pain. Much easier on Tyranical than Fortified. Take the trash slow and CC when you can.

Hard (Avoid these if you are looking to push keys or rush through) 

Temple of Sethralis

With all of its gimmicks and difficult mechanics, Temple of Sethralis is not a dungeon you run for its simplicity. Learn the boss fights, how to deal with the lightning ball gauntlets and placing the eye's of sethralis to be successful.

Shrine of the Storm

With every trash pull requiring some interrupt, CC or other mechanic, along with it's difficult bosses, Shrine of the Storm is one of the hardest dungeons. Make sure you understand the trash's abilities as well as how the boss fights work or you will have a hard time.

You can check out my Affix tier list and how to deal with them here.