Well, the correct answer is Caliber, because, the right name is Calibre .
To be serious, it depeneds on used technology: for nanometer techs, the widely used tool is Mentor Calibre. Synopsys Hercules is still used, but it is enough old and replaced by Synopsys IC Validator. Regarding Assura - it may be used for old technologies only, for example TSMC does not support it for 40nm.