As long as you can export to the required format which is irc. RAW16 (.r16) your fine with anyone of them. I prefer WorldMachine since its pretty straight forward, just a heightmap generator with no special bling bling. WorldMachine combined with ZBrush/Mudbox or if you need a free version Sculptris can produce anything you want. But if you need more features like skybox generation you better stick on Terragen or VUE. All of them have pretty much the same features but anyone of them is great for a different part. WorldMachine is brilliant for erosion effects, VUE is great for large terrains since its way more responsive and has tones of additional assets. Terragen 2 is something i've not tried yet but to be honest Terragen 1 was the #1 weapon of choice back in the days so im sure it has anything thats needed.
Hope this helps a bit in finding the right tool for you.
I would +1 WorldMachine, and as far as I'm aware, it's the best (if not the only) one to add roads and rivers to your terrain. That said, you'd be better off buying the standard version, as the 512x512 heightmap size limit of the free prevents you from taking full advantage of Landscape. Also, I have to say that world machine is designed to make big terrains, and its tools don't work that well for precision, or small scale work. Still worth it, though.
Geocontrol has roads AND rivers and lakes and the ability to make masks of all
Geocontrol combined with World machine is really a complete setup
L3DT is another option, very versitile. L3DT can generate a complex terrain from a simpler heightfield, layout a generalized water table (to calculate climates and texture alpha masks), generate normals, lighting, texturesplatting, etc. I've been looking at Geocontrol too, it has some sweet functions.
Bumping! It must have more tools out there, with decent UDK integration.
you can try this one http://www.lilchips.com/hmes.asp
used it a few times and intergrates with UDK nice and easy
+1 worldmahine i don't know about most of other softwares but i don't think that all of them provide texturing which is so cool
u can check what this dude have done with worldmachine and udk http://www.youtube.com/watch?v=p0o3bqoM0Qg
Personally, I do the basic terrain shapes that I am looking for in UDK by sculpting it, then I export the height map into World machine2. The reason why I use world machine is handling weathering effects, that are a pain in the arse to do by hand. I also use world machine to export several maps including flow maps, deposit maps and them combine them all in PS, then I import it all back into UDK and get something presentable rather face. From there its basically just setting up your landscape material to paint it
I pick things up, and I put them down!
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