Dragon has been around the longest and probably has the most complete transition set for terrain. The main limitation with Dragon is that it does not support 3-way terrain transitions, so that means that no matter what, you will probably need to go in and manually fix a lot of spots on the map (especially coastlines). I'm also pretty sure it doesn't support all the various maps (like Tokuno).
Map Generator 2 and UO Landscaper are very similar. Like Dragon, these programs make UO maps from image files. But instead of using just 1 basic image for the map, they instead use 2 (one .bmp for the terrain and one .bmp for the altitude). Among other things, this allows these programs to support 3-way transitions.
The main limitation for these programs is that the transition set is fairly complete but not 100% complete. Even though they support 3-way transitions, a lot of the 3-way transitions haven't been built. The good news for both programs is that there is nothing stopping you (or anyone) from building the transitions yourself.
Also, UO Landscaper suffers from an annoying bug that makes it impossible to render Map0.mul without a LOT of RAM (any system under a gig of memory will be impossible and Ive heard of people even with 2 gigs+ having trouble). Smaller maps seem to render ok. This bug has been around since the start of UOL and its really unknown when/if it will ever be fixed (although they have been promising a new version is forthcoming that will fix the bug).
Both Map Generator 2 and UO Landscaper use the same basic transition files (most of the existing Map Generator 2 transitions were converted from UO Landscaper).
My advice is to use Map Generator 2 if you are willing to toy around with creating/tweaking any missing transition sets. It is probably the best one out there right now, supports 3-way transitions, and punt is still actively developing and supporting it. It also supports all of the current map sizes (in fact you can actually define the map sizes so even if OSI creates a new map, Worldmaker should support it).
http://worldmaker.ryandor.com
However, if you want something simple, easy, don't want to fine tune transitions, BUT are also willing to go in and do a LOT of manual clean-up to your map using some other editing program like Worldforge, I would recommend Dragon. You can find Dragon here at Ryandor (I recommend using either Mod 9 or Mod 11 as the transition set, also available here).
UO Landscaper can be found over at Orbsydia:
http://www.orbsydia.com
Hope this info is helpful!
_________________
-= HellRazor =-
Shattered Sosaria is coming!
http://www.shatteredsosaria.com