For me personally, I'd probably say Death Mountain. It looks nice from a distance, but isn't very interesting to explore. The Gorons also have their most bland (blandest?) characterization in BotW, so even they don't make an already boring place more enjoyable to be in.
Hebra to me is the worst area in the game. Its a bit to big and you can't see very well in the region. It isnt very interesting as other areas and a bit hard to traverse. Its looks amazing with the sun out out but its pretty rare in Hebra to be sunny all the time. I did really like it at first but now its my least favorite region in BOTW. I hope they do more with it in BOTW II.
I may catch flack for this but I didn't care for the Akkala region. Sure, the red leaves gave it a nice autumnal vibe but there was absolutely nothing to be discovered there. Most of the region was taken up by those woods with the Ancient Guardians and it was just overall dull. It doesn't help that I rarely used to the Tech Lab and most of my memories from that region is annoyingly flying down to Talley Town.
I'm not a big fan of Hebra, like others have said, and Death Mountain is a bit of a one note area for my tastes. I like volcanoes, but there wasn't enough variety for me to really enjoy the area that much. Faron is annoying to navigate because of the terrible water mechanics, but the visual more than makes up for it. I think Death Mountain takes the cake for me though. I don't like Breath of the Wild's Gorons very much, particularly when compared to the Zora a couple regions over, the Divine Beast there is my least favorite, and there just isn't as much fun stuff to do there. Hebra would be worse, except I love shield surfing, and Hebra's great for it.
I agree with Gerudo Highlands. I enjoy the Hebra region more than most because it reminds me of snowshoeing back home, but the Gerudo Highlands are rather dull.
The other one is Death Mountain, which is really disappointing for me as the one area in the game you can see from pretty much anywhere. The lava makes exploring the area rather annoying, and when you do get the chance to explore you kinda realize everything looks the same and there isn't a ton to do. It's just not as interesting as it should be for such a major landmark in the world.