The current version is 1.8.6
The current version 1.8.6 is linked against the Subversion library 1.8.8.
Please make sure that you choose the right installer for your PC, otherwise the setup will fail.
To verify the file integrity follow these instructions.
If you're installing TortoiseSVN 1.8 on Windows XP SP3, you must have the latest msi installer installed. You can get that installer from Microsoft:
Windows Installer 4.5 Redistributable
If there is a bug in the latest released version which you really cannot live with, and which has already been reported as fixed, then you can try installing a pre-release build from the stable release branch.