First off I can't pin this on one sim so I put this in Hanger Chat as REX is for all platforms we use for the most part.  I can't quite nail this one down and I have the 'HD' version of REX.  I've seen some great screenshots out there but have not found the right marriage for both FS9 and P3D v2.3.  I settled on 'Bright' for a long time in FS9 until recently in watching YouTube videos found the upper level sight distance in the real world is always hazy with a ground sight distance of 150 miles or so depending on clouds (looking down from 20,000ft+).  I also got sick of the detail box around the aircraft that is visible at 30,000ft killing the experience in FS9 (I still use it along with P3D).  So I opted to do away with the 'Bright' daytime REX sky texture I've used for years in favor of something else.  Another issue with many of REX's daytime sky textures is haze in the distance is not chalky white looking.  Mountains in the distance are not white for the most part their a dark brown against the horizon.  That dark brown at altitude stretches to the horizon.  I've tried other Sky textures but REX seems to have gotten allot right, it's just picking the right one.  After adjusting haze to more accurately depict sight distance I settled on 'HIGH FLYER' in REX.

 

'High Flyer' gives that right balance of the darker tone of the ground going out to the horizon plus the gradual darkness as you climb higher in altitude.  Works well in both FS9 and P3D v2.  Problem is you can see the dark upper level when on the ground or taking off.  My question is what are people's favorite daytime sky texture?  Many of REX's skies look plain wrong yet there are allot of options.  It would be interesting to see what others have settled on and maybe I can save myself some trouble with searching (only looking at the most popular for starters).  Thanks in advance to all those who share what their using.

I generally keep my sky set to the Pacific and West Coast settings since I fly there most often (not sure what the exact settings are named off the top of my head) but I change clouds before every flight based on the weather conditions at my departing airport. Totally worth 60 seconds of my time. ASN does the rest.

 

I don't think there is a one-size-fits-all solution if you are aiming for realism, but I do very much enjoy the US west coast sky colors. Beautiful sunsets for sure. Sometimes I use the polluted sky if I'm flying socal. 

Since I'm in Texas, I use the summer heat. I'm using Texture Direct. Have you looked at the latest version? I found it to be a little better than the previous version.

I think the best sky textures all around for FS9, FSX, and P3D are the freeware HDEv2 sky textures. If the FSX version of these are the same format as what P3D uses I would stick with them.

I use Oceanic for the daytime sky texture. It just looks most natural to me, plus it includes a lighter/hazy appearance near the horizon. I use the Pacific (iirc) dusk/dawn textures but I will change those from time to time.