Good game for beginners

It doesn't really matter.
Grimdark Future and Age of Fantasy are essentially the same game with some differently flavored wording to fit the setting better and a handful of different special rules.
Firefight and Skirmish are broken down to play with far less models, but also has some additional mechanics to handle.
Regiments is the rank and file version of Age of Fantasy with modified unit coherency as fixed formations, unit facing and more limited movement, increasing the tactical depth drastically.
Warfleets is space ship battles, not unlike Battlefleet Gothic or Star Wars: Armada.


Personal suggestion would be Grimdark Future.
Start with the Basic Rulebook, it has all the concepts included to play Grimdark Future.
If you already familiar with the basic concepts of wargameing you can also simply use the Core Rules as Reference sheet. (Core Rules and Basic Rulebook differ slightly in some details, so make sure you both are using the same rules to avoid confusion)

For starter factions it is more like what you should avoid: Custodian Brothers, Human Inquisition, Prime Brothers, Titan Lords and Daemons.
Those factions either have a too expensive unit roster or are extremely oppressive at a beginner game.

For a beginner game I suggest the following:

• 1000 pts
  
• 4'x4' table
• 3 to 5 units
• max 1 hero
• only up to Tough(6) models
• No combined units
• No Aircraft

Build your army lists here: army-forge.onepagerules.com/
Get your army books as pdf here: webapp.onepagerules.com/