I'm currently working on a new design an I would like to try positioning a big slogan (not a baseline) right at the top of the page next to the logo.
On most websites, the slogan is vertically positioned almost in the middle of the viewport.
According to you, is the very top part of the screen a wrong place for a slogan?
It's a good idea to try to place it next to the logo at the top of the page. It help the user to understand what the brand and the site is all about.
I would also look for a brand identity manual about slogan placement. Usually there is a section about how to position the slogan relatively to the logotype.
Steve Krug discussed taglines in his book Don't Make Me Think. From his book:
"...the tagline appears right below, above, or next to the Site ID. (They) are a very efficient way to get your message across, because they're the one place on the page where users most expect to find a concise statement of the site's purpose."
However, he noted that a tagline is different from a motto and we need to be careful with them.
"Don't confuse a tagline with a motto, like 'We bring good things to life...' A motto expresses a guiding principle, a goal, or an ideal, but a tagline conveys a value proposition. Mottoes are lofty and reassuring, but if I don't know what the thing is, a motto isn't going to tell me."
If your slogan is a tagline, then the top of the page next to the logo is the best place for it. If it is a motto, then re-examine how much space you want to give it since it probably expresses some good things, but doesn't tell you anything about the site.
I think it depends on what you want to achieve with your slogan.
Brand enhancing slogans
If the main reason is brand familiarity then I would position it as close to the logo/branding element as possible, in a way that support.
For instance: Philips, sense and simplicity.
This will make people go, ow cool Philips makes things that makes sense and are simple to use...awesome. Another good example, I think, is Nike and Just do it.
Product enhancing slogans
These are slogans/taglines that add value to the product/service one is selling.
Philips Led: Brightens up your nights (I'm totally making this up but you get the point). This is directly related to the lightning business of Philips, e.g. product specific.
Nike fuel: Motivation on your wrist. Again a tagline that enhances the product/service.
These are usually found at the center of the page, near the product service that is sold and apart from the main branding of the company.