What game engine is best for a sandbox open-world game?

There's no "best" engine that you're going to want to use. Ask your programmers what they're familiar with.

There aren't any engines that are really optimized for an open-world game. That said, it's probably going to be easier to use an existing engine rather than, for example, rewriting model-and-animation file loading code.

Every engine you've mentioned has been used for open world games in the past. What you might want to consider instead is exactly which features you're going to need to implement your open world. A Skyrim style game with static terrain is very different from a Minecraft fully-editable world which is in turn very different from an MMO-style regional map, which is in turn different from a more strategic game.

I'd guess you need to look at a way to handle dividing the content into sections, paging them in and out of the world as the player travels. What happens when the player isn't there? Is this the kind of game where you can simply freeze the enemies when you are far enough away, or do you have a Far-Cry-4-like animal migration system? Or do some things need to be remembered, while other things can just be despawned? How far is your draw distance?

Answering technical questions like these in terms of your design and engineering plan will tell you a lot more about what you need to be looking for in an engine, and what kind of features your team will need to implement yourselves.

Only because I see the term sandbox. You'll be better off building a game engine yourself. Most of the engines currently availible aren't optimized for what you are probably thinking of. If you're doing something like minecraft, it's not too hard.

If it's something like Arma or Far Cry, don't bother.

in a recent thread, i mentioned that unity doesn't seem to have built-in support for large continuous worlds with no load screens.

someone replied that one of the other engines apparently does. i think it was the unreal engine, or maybe the CRY engine, can't recall.

i'm currently working on three large continuous world titles, and am planning two more after those. so engines for large continuous worlds is a topic of continuing interest to me.

but the fact is, i've gotten far enough along in the first title that i now have the in-house library/engine/code necessary to just do all five with no third party engine - but not necessarily with all the whiz-bang stuff some engines can do.

in a recent thread, i mentioned that unity doesn't seem to have built-in support for large continuous worlds with no load screens.

someone replied that one of the other engines apparently does. i think it was the unreal engine, or maybe the CRY engine, can't recall.

i'm currently working on three large continuous world titles, and am planning two more after those. so engines for large continuous worlds is a topic of continuing interest to me.

but the fact is, i've gotten far enough along in the first title that i now have the in-house library/engine/code necessary to just do all five with no third party engine - but not necessarily with all the whiz-bang stuff some engines can do.

Well, Unity does have LoadLevelAdditiveAsyc which is a foundation for streaming levels continuously (plus some of the Pro features to help with this are now free in 5). Unreal has level streaming. It's not that there's no support. It's that the real question is what kind of data loading does a particular game actually need.

I'm using unreal engine 4 to create an open world role-playing game. I chose it because it has level streaming and large map capabilities plus terrain editing out of the box at no extra cost amongst other things.

Evaluate it amongst other things and find something that works well for you...

Unigine claims to have explicit support for large streaming worlds, but the engine is not as cheap or widely known as Unity and Unreal.

Oh yeah... there is the Cryengine, but you need to pay monthly for it. And you don't get all of the source code.

Unreal Engine might be able to do it. But you might have to add the feature yourself.

Unreal Engine might be able to do it. But you might have to add the feature yourself.

Nah everything you need is generally built in :)

Unreal must be the one they were talking about. they said everything was in there, and it all just works straight out the gate supposedly.

sounds like i ought to be taking a look at that one.