Source Engine is pretty much the best engine made. It has no bugs and works 100% of the time, not to mention it has a powerful dynamic lighting system and dynamic level loading for seamless transitions. However, let's talk about some of its other, even more impressive features:

-Baked lighting that never screws up: All terrain in the Source Engine is made up of sheets called "displacements," which are simply flat surfaces subdivided into triangles, and then warped to give a more natural and curved appearance. Because of Source's inability to ever screw up, ugly black splotches and rainbows never ever ever randomly appear on displacements for no reason. Totally never happened on any officially released maps either.

Oh, and it also has a neat feature where sometimes if you put the origin of an entity inside of a block, the entire map doesn't go pitch black! I betcha Unreal can't do that.

-Easy to use model compilers: Compiling models for Source is a painless, one-click process. All you have to do is make a text file that feeds into a command line-based compiler and give it absolute file paths typed manually, and you're good to go! You also never have to use 3rd party tools as the only way to get custom TF2 weapons into the game.

-Map compiling tools that works like a charm: Ever feared that your compiling tools would ever stop working randomly and require a full restart of Steam to get working again? Don't worry, Source never does that either!

-A powerful map editor: Source's map editor, Hammer, is an incredibly powerful and stable tool. Boasting an impressive crash-prevention system, you'll never have to worry about your tools crashing every time you do anything. It's especially great in mappers don't develop nervous tics and press Ctrl+S every 30 seconds!

What's even more impressive is how Hammer handles "invalid solids." An invalid solid is where a solid has a face where all of its points aren't coplanar, meaning the face can't be drawn properly. Hammer handles these in an incredibly graceful manner, in which it doesn't delete every entity in your map when you load a map containing an invalid solid! Impressive work, Valve.

-Up to date: Source wasn't created in 2004 at all. It's totally not been improved drastically to keep up with today's standards, and the asset creators totally don't have to compensate for how bad the engine is at all!

tl;dr: