I have used all stacks, and I still don’t have a favorite one to stick with, because each design has its mood that can be used with certain niches/industries.
For example:
Integrity
This stack is universal. An example where I have used it is http://www.outwardboundcroatia.com/en/. I guess I didn’t want to look too niche, because the target audience is very broad. I have tried other stacks, but simply I felt this one suits the site best.
Renew
Renew is modern and flat, and it is a versatile stack. For example, I have used it with http://www.dentagra.hr/, as it is for a company that works with high-tech devices, and providing CRM. Renew really suits this, but also a host of other industries. It is also a good base for heavy customization, so I have used it here too: http://www.brankavalcic.com/.
I have also used it with my latest project made for the government: https://www.discoverplitvice.com/. Because it doesn’t have strong personality, it helps the subject, UNESCO protected World Heritage - Plitvice Lakes National Park, to stand in the front row.
There are more projects where I have used Renew, so it is my most used stack. Not the favorite, just most used based on project needs.
Icon
Icon is underestimated. This great stack adds personality. It is minimal, allowing design and UX to come out to the spotlight. I have used it with http://www.mojemalozlato.com/en/, a gluten-free bakery. I really wanted to bring out the branding and personality of this business, and Icon was the best possible choice.
Ethos
Ethos is serious and graceful. I have used it on a corporate site that rivals local banks and other financial institutions, https://www.progreso.hr/. Also the blog layout of this stack is beautiful and functional, and that was very important for this site.
I usually know the stack I will use before even starting the project. But I still like to switch them in the earlier dev phase, to see how would a particular layout look like in each. Once I am confident that I’ll stick with a particular stack, then I start with the customization of various elements.