It depends on what you're used to.
I came to Git from a Perforce world and as such, found P4Merge the best. It is free and easy to setup with Git.
However, I soon found P4Merge limiting and quickly graduated to BeyondCompare, which is not free.
If you're new to merging, start simple and try P4Merge.
UPDATE: I have now stopped using Beyond Compare and use KDiff3 instead. The only thing you need to use kdiff3 are the keyboard shortcuts.
The panels you see in a 3 way merge are numbered 1, 2 and 3 from left to right.
To resolve a merge conflict by choosing the changes from panel 1, you hit Ctrl-1. Hit Ctrl-1 again and it turns off the change from panel 1. If you want changes from panel 1, followed by panel 2, hit Ctrl+1, then Ctrl+2. If you want them in the reverse order, you hit Ctrl+2 then Ctrl+1. See where I'm going with this? Its incredibly simple.
Finally, you navigate between conflicts with Ctrl-Up and Ctrl-Down. If you want to manually add in changes, you just type. That's it.
Finally, as a nice bonus, if you're using Git Extensions on Windows, it comes with built in support for KDiff3.
PS> Even if you like the command line, you can invoke a GUI using