Honestly WordPress is a really good option... Last market share I saw WordPress was 54% of the websites of the internet.
Other options would be:
- Joomla( as mentioned above) - It's okay, it definitely requires more skill than wordpress and there are less theme options.
- Drupal - Similar to Joomla, where it's more advanced, but it's not really as much e-commerce focused as Joomla is.
The nice thing about WordPress and Joomla is most webhosts will have autoinstallers for them.
So when you're installing WordPress on 15 domains it's definitely a time saver.
I would go with WordPress myself.