Ideally I would like to have the tikzpictures be in one line centered horizontally with its caption each preserving its node size and line strength and vertically aligned to a common bottom line. I am quite stuck however - therefore all help is much appreciated as I simply have no time to work through the subcaption manual (if a better alternative is out there, a hint is appreciated too) and definitely not pgf manual documentation right now - though the pgf manual is on my todo list for sure.
\documentclass[a4paper, twoside]{book}
\usepackage{subcaption}
\usepackage{tikz}
\usetikzlibrary{shapes,backgrounds,shapes,positioning,petri,topaths}
\begin{document}
\begin{figure}[ht]
\centering
\begin{subfigure}[b]{0.16\textwidth}
\begin{tikzpicture}[scale=0.50]
\node (v1) at (2,3) [circle, draw] {};
\end{tikzpicture}
\caption{$K_1$}
\label{fig:20140812k1}
\end{subfigure}%
%\quad
\begin{subfigure}[b]{0.3\textwidth}
\begin{tikzpicture}[scale=0.50]
\node (v1) at (2,1) [circle, draw] {};
\node (v2) at (2,3) [circle, draw] {};
\draw (v1) -- (v2);
\end{tikzpicture}
\caption{$K_2$}
\label{fig:20140806k2}
\end{subfigure}
\begin{subfigure}[b]{0.16\textwidth}
\begin{tikzpicture}[scale=0.50]
\node (v1) at (1,1) [circle, draw] {};
\node (v2) at (3,1) [circle, draw] {};
\node (v3) at (2,3) [circle, draw] {};
\draw (v1) -- (v2);
\draw (v2) -- (v3);
\draw (v1) -- (v3);
\end{tikzpicture}
\caption{$K_3$}
\label{fig:20140812k3}
\end{subfigure}%
%\quad
\begin{subfigure}[b]{0.16\textwidth}
\begin{tikzpicture}[scale=0.50]
\node (v1) at (1,1) [circle, draw] {};
\node (v2) at (5,1) [circle, draw] {};
\node (v3) at (3,2) [circle, draw] {};
\node (v4) at (3,4) [circle, draw] {};
\draw (v1) -- (v2);
\draw (v1) -- (v3);
\draw (v1) -- (v4);
\draw (v2) -- (v3);
\draw (v2) -- (v4);
\draw (v3) -- (v4);
\end{tikzpicture}
\caption{$K_4$}
\label{fig:20140812k4}
\end{subfigure}%
%\quad
\begin{subfigure}[b]{0.16\textwidth}
\begin{tikzpicture}[scale=0.50]
\node (v1) at (2,1) [circle, draw] {};
\node (v2) at (4,1) [circle, draw] {};
\node (v3) at (1,3) [circle, draw] {};
\node (v4) at (5,3) [circle, draw] {};
\node (v5) at (3,4) [circle, draw] {};
\draw (v1) -- (v2);
\draw (v1) -- (v3);
\draw (v1) -- (v4);
\draw (v1) -- (v5);
\draw (v2) -- (v3);
\draw (v2) -- (v4);
\draw (v2) -- (v5);
\draw (v3) -- (v4);
\draw (v3) -- (v5);
\draw (v4) -- (v5);
\end{tikzpicture}
\caption{$K_5$}
\label{fig:20140812k5}
\end{subfigure}%
%\quad
\begin{subfigure}[b]{0.16\textwidth}
\begin{tikzpicture}[scale=0.50]
\node (v1) at (2,1) [circle, draw] {};
\node (v2) at (4,1) [circle, draw] {};
\node (v3) at (1,3) [circle, draw] {};
\node (v4) at (5,3) [circle, draw] {};
\node (v5) at (2,5) [circle, draw] {};
\node (v6) at (4,5) [circle, draw] {};
\draw (v1) -- (v2);
\draw (v1) -- (v3);
\draw (v1) -- (v4);
\draw (v1) -- (v5);
\draw (v1) -- (v6);
\draw (v2) -- (v3);
\draw (v2) -- (v4);
\draw (v2) -- (v5);
\draw (v2) -- (v6);
\draw (v3) -- (v4);
\draw (v3) -- (v5);
\draw (v3) -- (v6);
\draw (v4) -- (v5);
\draw (v4) -- (v6);
\draw (v5) -- (v6);
\end{tikzpicture}
\caption{$K_6$}
\label{fig:20140812k6}
\end{subfigure}%
\caption{Vollständige Graphen $K_1$ bi $K_6$}\label{fig:completeGraphs}
\end{figure}
\end{document}

\centeringafterbegin{subfigure}and before\begin{tikzpicture}and that should solve the horizontal-alignment issue. – Adam Liter Aug 12 '14 at 19:29