Best free base themes for freelance developer?

Hi,

I recently started making a few websites in WordPress in my spare time. Business has been doing quite well for me, making inexpenssive websites for small companies etc. Although i’m not a professional I want my product to be as professional as possible so I’ve been searching for a solid base theme for my projects but I’m struggling to find one suitable for my needs. I’m looking for a theme with the following features listed by priority:

• Responsive
• SEO optimized (as much as a theme can be)
• Clean minimal design – I usually do the designs myself
• Clean code (valid) and fast
• Lots of widget areas to emulate a Joomla-like level of customization
• Useful shortcodes (especially tab groups and accordion lists)

I’ve built sites with Hybrid, Thematic and Skeleton. It could just be a coincidence but Hybrid is the only theme that has gotten the company as the first search result on Google when searched for the company’s name. That includes all the sub-pages on the site and I did NOT do a lot of SEO on that site in comparison to the two others where the sites (fornt page only) show up somewhere on the bottom of the first page.

I tried Gantry which seemed absolutely amazing and packed with features but also bloated with excessive code because of all the features I think. I visited a few sites using the framework and they were all incredibly slow and scored extremely poorly in Google’s PageSpeed. I also tried Attitude, Basic by Themify, Customizr, Foundation, Quark, Roots, Shell (from ThemeHybrid like Hybrid but I don’t know how it is SEO-wise and I haven’t seen a lot of feeback) and WhiteBoard. Maybe I missed something or maybe one of the themes I tried are perfect for my needs, I just need some recommendations 🙂