Simple table with caption:

\begin{longtable}[c]{@{}rlcl@{}}
\hline\noalign{\medskip}
Right & Left & Center & Default
\\\noalign{\medskip}
\hline\noalign{\medskip}
12 & 12 & 12 & 12
\\\noalign{\medskip}
123 & 123 & 123 & 123
\\\noalign{\medskip}
1 & 1 & 1 & 1
\\\noalign{\medskip}
\hline
\noalign{\medskip}
\caption{Demonstration of simple table syntax.}
\end{longtable}

Simple table without caption:

\begin{longtable}[c]{@{}rlcl@{}}
\hline\noalign{\medskip}
Right & Left & Center & Default
\\\noalign{\medskip}
\hline\noalign{\medskip}
12 & 12 & 12 & 12
\\\noalign{\medskip}
123 & 123 & 123 & 123
\\\noalign{\medskip}
1 & 1 & 1 & 1
\\\noalign{\medskip}
\hline
\end{longtable}

Simple table indented two spaces:

\begin{longtable}[c]{@{}rlcl@{}}
\hline\noalign{\medskip}
Right & Left & Center & Default
\\\noalign{\medskip}
\hline\noalign{\medskip}
12 & 12 & 12 & 12
\\\noalign{\medskip}
123 & 123 & 123 & 123
\\\noalign{\medskip}
1 & 1 & 1 & 1
\\\noalign{\medskip}
\hline
\noalign{\medskip}
\caption{Demonstration of simple table syntax.}
\end{longtable}

Multiline table with caption:

\begin{longtable}[c]{@{}clrl@{}}
\hline\noalign{\medskip}
\begin{minipage}[b]{0.13\columnwidth}\centering
Centered Header
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright
Left Aligned
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedleft
Right Aligned
\end{minipage} & \begin{minipage}[b]{0.30\columnwidth}\raggedright
Default aligned
\end{minipage}
\\\noalign{\medskip}
\hline\noalign{\medskip}
\begin{minipage}[t]{0.13\columnwidth}\centering
First
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright
row
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedleft
12.0
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright
Example of a row that spans multiple lines.
\end{minipage}
\\\noalign{\medskip}
\begin{minipage}[t]{0.13\columnwidth}\centering
Second
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright
row
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedleft
5.0
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright
Here's another one. Note the blank line between rows.
\end{minipage}
\\\noalign{\medskip}
\hline
\noalign{\medskip}
\caption{Here's the caption. It may span multiple lines.}
\end{longtable}

Multiline table without caption:

\begin{longtable}[c]{@{}clrl@{}}
\hline\noalign{\medskip}
\begin{minipage}[b]{0.13\columnwidth}\centering
Centered Header
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright
Left Aligned
\end{minipage} & \begin{minipage}[b]{0.14\columnwidth}\raggedleft
Right Aligned
\end{minipage} & \begin{minipage}[b]{0.30\columnwidth}\raggedright
Default aligned
\end{minipage}
\\\noalign{\medskip}
\hline\noalign{\medskip}
\begin{minipage}[t]{0.13\columnwidth}\centering
First
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright
row
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedleft
12.0
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright
Example of a row that spans multiple lines.
\end{minipage}
\\\noalign{\medskip}
\begin{minipage}[t]{0.13\columnwidth}\centering
Second
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright
row
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedleft
5.0
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright
Here's another one. Note the blank line between rows.
\end{minipage}
\\\noalign{\medskip}
\hline
\end{longtable}

Table without column headers:

\begin{longtable}[c]{@{}rlcr@{}}
\hline\noalign{\medskip}
12 & 12 & 12 & 12
\\\noalign{\medskip}
123 & 123 & 123 & 123
\\\noalign{\medskip}
1 & 1 & 1 & 1
\\\noalign{\medskip}
\hline
\end{longtable}

Multiline table without column headers:

\begin{longtable}[c]{@{}clrl@{}}
\hline\noalign{\medskip}
\begin{minipage}[t]{0.13\columnwidth}\centering
First
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright
row
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedleft
12.0
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright
Example of a row that spans multiple lines.
\end{minipage}
\\\noalign{\medskip}
\begin{minipage}[t]{0.13\columnwidth}\centering
Second
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright
row
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedleft
5.0
\end{minipage} & \begin{minipage}[t]{0.30\columnwidth}\raggedright
Here's another one. Note the blank line between rows.
\end{minipage}
\\\noalign{\medskip}
\hline
\end{longtable}