This is a theme I enjoy, pun intended.
First I just want to say that all the built-in themes are very capable and highly functional.
That said, it really depends. If this is for a business app, I often try to use one that best resembles the colors, logo or design vive of the company. So, I probably wouldn't use a theme with green hues if the logo is red (unless it's a Christmas look and feel I'm going for). Then the other criteria may be the amount of data to be displayed by the applications. Some themes have a lot of white space and it would be challenging to fit many fields and report columns in them.
Sometimes, I know the theme needs to be customized to match a corporate look. I may go with a theme that gets me half way there. But sometimes I will simply start creating a theme from scratch. This may sound scary, but it's not that terrible. Even from scratch APEX will create certain skeleton elements for you. This approach certainly requires someone to be comfortable with HTML and CSS.
If you're working with someone that will dictate design and be controlling about element placement, then perhaps I would recommend you do not go with Theme 25. Theme 25, Responsive, does require understanding that screens will look different at different browser sizes (resolutions). The site you get on a mobile phone is very different from what you will see on a tablet or a desktop. Theme 25 is great, but you better be ready to "go with the flow" or you're going to bump heads a lot.
In other hand, if you want a site that is pretty decent on a smart phone and on the desktop, Theme 25 is excellent even without the need to do jQuery Mobile specific pages and elements.
I hope this gives you some ideas.
Thanks for responding.
I'm new to apex. So currently it is not an option to customize a theme.
So I restrict my question: theme 25 or theme 26?
For a small example see this entry Re: APEX Theme 25 - Responsive Grid Layout Question
I feel inclined to just recommend Theme 26. But if your application requires the responsiveness by all means go with 25.
PS. I'm working on a series of blog posts related to theme 25 layout.