How to create custom panel point everywhere in community

Explicit Panels are not included in the navigation.  Only IContainerPanel, IApplicationPanel and IThemePanel defined panels will be added to the navigation.

What is proper panel type - to create a top level item, in a Manage left-side-bar, so that item will be available from any place in Verint?

Top level panels cannot be added.  You can only add panels with the interfaces listed above and they will be displayed in the appropriate categories based on those implementations.  What is your panel's use case, maybe that can assist us in providing the best option to implement it?

We are developing SEO plugin that will provide possibility to define meta tags in every part of site. 

A Container Panel would be available on every (with the exception of a few site level pages), it would just be under Manage Group.  A Theme Panel could be available on all pages, just under Manage Site Theme, Manage Group Theme, etc.  

Another option would be a UI widget that exists on every page, but only visible to users who have access to the SEO panel, that link would directly open the explicit panel.

Mathew D'Amico is it possible to render widgets on that panel?

You can use a widget to render the panel contents.  Using Widgets to Render Content From Plugins  

We had a similar use-case and ended up putting our extra panels into IThemePanel (so they appear in Manage Group Theme, etc).. it's not perfect, but at least they're available everywhere that we need them without putting a UI widget on all pages (which feels like a messy workaround).