The conundrum here is that the request is for a terrain
scenery that "covers the entire planet." To my knowledge there are very few enhancements that are truly global and those that are are usually just texture replacements for the stock FSX product. The really significant terrain sceneries are targeted to specific regions to improve accuracy and detail; think of this as "quality" over "quanity."
Second, while a targeted product like Ultimate Terrain can provide several types of enhancements (ie, improved landclass, new textures and some polygon objects) under one under one umbrella, your typical 'global' enhancement can only focus on one aspect (ie, terrain mesh).
That having been said, here are some 'entire planet' terrain enhancements you might consider:
1. FS Genesis World Terrain Mesh
-- This product improves the shape of mountains and valleys throughout most of the world. I haven't explored the default FSX world yet but addon meshes have long been a necessity for previous versions of FS to present regions like the Alps in their full glory. FS Genesis also has separate products that cover regions North America
2. FS Global
-- Another mesh product. Read Andy Herd's review to help you decide if a mesh replacement is where you want to invest your money and/or hard drive space.
3. MyWorld Landclass
-- The accuracy of landclass replacements are largely hit-or-miss in my opinion, but you will
see a noticeable change in how your terrain textures are distributed. I used MyWorld in FS9 and I was generally satisfied with it because it made much of the globe visually more interesting; however, since I have never been out of North America I can't verify the authenticity of MyWorld's enhancements in general. With FS9 I used to apply FS Genesis's US Landclass Project over MyWorld because I felt it was superior for representing the USA, but overall I was never completely satisfied with either product's representation of Montana and surrounding states.
4. Scenery Tech FSX landclass
-- I'm currently using the North America product and I think it is an adequate improvement for most of the USA. However, I quicky noticed that Erie, Pennsylvania (an industrial city where I have lived) was rendered incorrectly as suburban/rural. It seems that this product is conservative in its representation of population centers; see this AVSIM review
. I found that giving Ultimate Terrain X a higher priority in the scenery library helps to make the larger cities look more appropriately urban.
5. Ultimate Terrain X
-- I'm a huge fan of the Ultimate Terrain series for FS9. Out of the box, FSX brings improved default coastlines to the FS series so the impact of UTX is seemingly less impressive than the original UT product. However, UTX does quite a lot and shouldn't be dismissed. Railroads, minor roads, lakes, rivers, cemetaries and parks are added; coastlines are tweaked; night lighting is improved; metropolitan areas are more accurate. UTX even includes options for the color of your water. IMO, UTX is the best alternative to photoscenery if you are flying VFR.
6. FS Scene
-- Texture replacements galore including a total package for FSX. I have no experience with this product line.
7. Ultimate Night X
-- Another product I've never used.