Which Transformers are most in scale with GI Joe?

There really aren't any/many TFs in scale with GI Joe. There have been a few motorcycles over the year that were in scale in vehicle mode, but their bot modes were tiny (compared to what they 'should' be based off TF sizes). There are some third-party and knock off Transformers that are in scale, but most are just too small to work well with Joes.

I primarily mean the vehicle mode. But while were at it did anyone ever make a tutorial for making Joe vehicles that transform

If you want vehicle mode, then look at the motorcycles. Also look for the knock off oversized Masterpiece Lambor. He's 1:18. Also some of the Roadbots (bad bot modes but nice vehicles) would work. If you don't care about bot mode, maybe just regular vehicles with Transformers paying jobs?

For customizing, there have been a few here, but you're better off looking at a TF board for that. Get some leader or masterpiece figures, Joe vehicles, study the transformations and bash together based on that.