Meld is one of the best-looking and most intuitive merge tools I have ever tried. You just have to try it to believe it.
The user interface really shines, and merging is really intuitive: just point and click. Scroll around, merging is fun with Meld!
In the end, Meld is good and fun but I prefer KDiff3. KDiff3’s UI isn’t as pretty as Meld, but KDiff3 has good automatic resolution and it has more features too (that I seldom use, though.) XXDiff is probably the most powerful differ/file merger but it seems too complicated for me.