One option is to update \chapter and insert a regular \label. The structure should be something that you're not going to use elsewhere in the document, of course, since \labels have to be unique. I've chosen to insert \label{chapter-\thechapter}, which is similar to \label{chapter-2} in Chapter 2 if \thechapter is \arabic{chapter} (the defaut).
To insert this label, we redefine \chapter and use some refcount to extract it from the default equation number - (\thechapter.\arabic{equation}) within \chapref:

\documentclass[oneside]{book}
\usepackage{amsmath,hyperref,refcount}
\usepackage{blindtext}
\AtBeginDocument{
\let\oldchapter\chapter
\renewcommand{\chapter}[1]{%
\clearpage
\oldchapter{#1}% Regular chapter
\label{chapter-\thechapter}% \label this chapter
}
}
\makeatletter
\def\extract@chapter#1.#2\@nil{#1}%
\newcommand{\chapref}[1]{{%
\edef\x{\noexpand\edef\noexpand\chapnum{\noexpand\extract@chapter\getrefnumber{#1}\noexpand\@nil}}\x%
\ref{chapter-\chapnum}%
}}
\makeatother
\begin{document}
\chapter{something}
\blindtext[1]
\chapter{something else}
\section{a section}
This is the most important equation in this chapter
\begin{equation}
1=1 \label{eq:important-equation}
\end{equation}
\chapter{chapter 3}
As we have seen in Equation~\eqref{eq:important-equation} in Chapter~\chapref{eq:important-equation} on page~\pageref{eq:important-equation}, we have something important
\end{document}
The above solutions assumes you'll only use numbered chapters. However, if you're also using unnumbered chapters, then you can use the following redefinition of \chapter:
\usepackage{xparse}
\AtBeginDocument{
\let\oldchapter\chapter
\RenewDocumentCommand{\chapter}{s o m}{%
\clearpage
\IfBooleanTF{#1}
{\oldchapter*{#3}}% \chapter*[..]{...}
{\IfValueTF{#2}
{\oldchapter[#2]{#3}}% \chapter[..]{...}
{\oldchapter{#3}}% \chapter{...}
\label{chapter-\thechapter}% \label this chapter
}%
}
}
The only difference is the conditioning on whether or not you use \chapter or \chapter*, together with the conditional placement of \label.
Another solution, not compatible with amsmath:
zref is an easy option for this as it allows the user to specify more than the single \ref value via "properties".
Below I've created a new property chapter, which stored \thechapter. Additionally, each \chapter command has been redefined to insert a \label to allow for an appropriate hyperlink. Finally, \label is also updated to store the current chapter its used within.
If you just want the chapter number, and not a hyperlink to the start of the chapter, then there is no need to redefine \chapter (...life would be easier).

\documentclass[oneside]{book}
\usepackage{zref,hyperref}
\usepackage{blindtext}
\makeatletter
\zref@newprop{chapter}{\thechapter}% Chapter property holds \thechapter
\AtBeginDocument{%
\let\oldlabel\label
\renewcommand{\label}[1]{%
\zref@labelbyprops{#1}{chapter}% Special label
\oldlabel{#1}% Old label
}%
}
\let\oldchapter\chapter
\renewcommand{\chapter}[1]{%
\clearpage
\oldchapter{#1}% Regular chapter
\oldlabel{chapter-\thechapter}% \label this chapter
}
\newcommand{\chapref}[1]{%
\begingroup\edef\x{\endgroup\noexpand\hyperref[chapter-\zref@extract{#1}{chapter}]{%
\zref@extract{#1}{chapter}}}\x%
}
\makeatother
\begin{document}
\chapter{something}
\blindtext[1]
\chapter{something else}
\section{a section}
This is the most important equation in this chapter
\begin{equation}
1=1 \label{eq:important-equation}
\end{equation}
\chapter{chapter 3}
As we have seen in Equation~\ref{eq:important-equation} in Chapter~\chapref{eq:important-equation} on page~\pageref{eq:important-equation}, we have something important
\end{document}
Some related references:
varioref? – Jérôme Dequeker Jan 18 '16 at 16:35nameref. See : http://tex.stackexchange.com/questions/6238/get-the-title-instead-of-the-number-of-a-referenced-chapter-section – Jérôme Dequeker Jan 18 '16 at 16:51cleveref. See : http://tex.stackexchange.com/questions/109843/cleveref-and-named-theorems – Jérôme Dequeker Jan 18 '16 at 16:55namerefpackage you are able to have the chapter name, right ? and with thevariorefpackage, you have the chapter first page ?So I don't understand where the problem is.
– Jérôme Dequeker Jan 18 '16 at 17:35refgets updated too. I hope that now is more clear. – dadexix86 Jan 18 '16 at 17:44\pageref{important-equation}. for chapter, I will see. – touhami Jan 18 '16 at 17:47