Hi. I am the owner of several offline stores. Now I want make my business online. I am try different online website builders, but functionals very simple. Can you advice me website builder or CMS with huge functionality and seo friendly modules.

Can you describe the desired functionality?
General functional you can find in Joomla with virtuemart modules. You can also use full package modules like a KALLYAS. Joomla have one drawback this is vulnerability to viruses.

I've used opencart a few times. Straighforward to use.
Free too.

Go for magento platform, it is the best platform used to build ecommerce website. it contaions many installed plugins which help to build best shopping cart.

Go for magento ecommerce platform. Magento is the best platform to build an online store. Moreover, it can be downloaded for free and has scalable platform. Besides this, it has countless extensions that helps in grabbing the audience and increase in website conversion rate.