I’ve noticed over the years it seems like Toyota feels that the answer to your question is a Barcelona Red Off-Road. If you look at the brochures and advertising since the 2014 refresh, Toyota has used this trim and color A LOT in its advertising. A red Trail/Off-Road has been the brochure cover photo every year since 2014. Shortly after the 2014 update, Toyota ran a "Keep It Wild" ad campaign that featured a Barcelona Red Trail. If you go to Toyota's website right now and choose 4Runner, there is a video at the top which features a Barcelona Red Off-Road. You can find Barcelona Red 4Rs all over the place in Toyota ads.
So, it looks like Toyota sees the Barcelona Red Off-Road as the quintessential 4Runner look.
But if the OR trim isn't your thing, I think it largely depends on which trim you’re considering. The different bumper colors, grills, and wheels make a big difference. Here are my thoughts:
SR5: silver or gray. Both of these colors tend to hide/compliment the more basic looks of SR5.
Off-Road: red or white. They both show off the vehicles lines and nicely complement the silver front and rear bumper trim.
Limited: white. The Limited is going for a fancier look and the white compliments the chrome.
Limited Nightshade: gray. Looks badass. Probably the best Limited combo/look of all.
Pro: white. The white is a great contrast to all the black pieces and wheels. Makes for a mean, Storm Trooper look.