I wasn't particularly happy with TortoiseSVN's diff and merge tool so I decided to look for an external tool. I came across DiffMerge from SourceGear, a maker of another version control system.  Here's a howto with how to use DiffMerge with TortoiseSVN as taken from here (copied for readability reasons):

By default tortoise installs its own diffviewer, tortoisemerge, for viewing and modifying changes. I prefer another. Sourcegear diffmerge provides a better user experience and the text is a lot easier to follow. To set tortoisesvn to use a different diffviewer we open settings and select diff viewer in the External Programs subtree. Select External radiobox.

Enter this text into the dialog box:

"C:\Program Files\Sourcegear\DiffMerge\DiffMerge.exe" /t1=mine /t2=theirs %mine %theirs

Click on Merge Tool and select External radiobox. Enter this into the textbox:

"C:\Program Files\Sourcegear\DiffMerge\DiffMerge.exe" /t1=mine /t2=base /t3=theirs /r=%merged %mine %base %theirs

Save and close the dialogbox. Fire up a file that was modified to compare and you'll be using Diffmerge instead of tortoisemerge.