You can use makecell, which allows for line breaks in cells, and hhline to have nicer intersection double lines, or use some colour in the table.
Here are aretwo solutions:
\documentclass{article}
\usepackage{hhline}
\usepackage{makecell, caption, booktabs}
\usepackage{siunitx}
\usepackage[table, svgnames]{xcolor}
\colorlet{tbgcolour}{Gainsboro!70!Lavender!50! white}
\captionsetup{font=small}
\begin{document}
\begin{table}[!htb]
\centering\sisetup{table-format=1.8, table-number-alignment=center}
\setlength{\extrarowheight}{2.5pt}
\caption{Actors vs Positional Dynamicity (based on Closeness Centrality) in Different LSNs} \label{pos1}
\begin{tabular}{|c||*{3}{S|} c|S|}
\hhline{-||-----|}
\makecell{Actor\\ No. } & {LSN 1} & {LSN 2} & {LSN 3} & ... & {LSN 60} \\ [0.5ex]
\hhline{=::=====}
1 &0.205602 &0.262515 & 0.247979 &... & 0.170467 \\
\hhline{-||-----|}
2 &0.00961852 &0.0200901 & 0.0207746 &... & 0.107013 \\
\hhline{-||-----|}
3 &0.1271 & 0.170967 & 0.199928 &... &0.173208 \\
\hhline{-||-----|}
4 & 0.00263733 & 0.00524802 & 0.00787202 &... &0.0100803 \\
\hhline{-||-----|}
5 & 0.00300429 &0.00597824 & 0.00673418 &... & 0.00706005 \\
\hhline{-||-----|}
... & ... & ... & ... &... . &... \\
\hhline{-||-----|}
1899 &0.00994494 &0.00968842 & 0.0216411 &...&0.0610909 \\
\hhline{-||-----|}
\end{tabular}
\end{table}
\begin{table}[!htb]
\centering\sisetup{table-format=1.8, table-number-alignment=center}
\caption{Actors vs Positional Dynamicity (based on Closeness Centrality) in Different LSNs} \label{pos1}
\arrayrulecolor{DarkOrange!50}
\setlength{\arrayrulewidth}{0.6pt}
\setlength{\extrarowheight}{2.5pt}
\begin{tabular}{>{\columncolor{tbgcolour}[\tabcolsep][\tabcolsep]}c!{\hspace{0.5ex}}*{3}{|S}|c|S|}%
\rowcolor{tbgcolour} \multicolumn{1}{c!{\color{white}\vrule width 4.4pt}}{\makecell{Actor\\ No.}} & \multicolumn{1}{!{\color{tbgcolour}\vrule\hspace{-2\arrayrulewidth}}c}{LSN 1} & \multicolumn{1}{c}{LSN 2} & \multicolumn{1}{c}{LSN 3} & \multicolumn{1}{c}{...} & \multicolumn{1}{c}{LSN 60} \\
\addlinespace[1ex]
\cline{2-6}
1 & 0.205602 &0.262515 & 0.247979 &... & 0.170467 \\
\cline{2-6}
2 & 0.00961852 &0.0200901 & 0.0207746 &... & 0.107013 \\
\cline{2-6}
3 & 0.1271 & 0.170967 & 0.199928 &... &0.173208 \\
\cline{2-6}
4 & 0.00263733 & 0.00524802 & 0.00787202 &... &0.0100803 \\
\cline{2-6}
5 & 0.00300429 &0.00597824 & 0.00673418 &... & 0.00706005 \\
\cline{2-6}
... & ... & ... & ... &... . &... \\
\cline{2-6}
1899 & 0.00994494 &0.00968842 & 0.0216411 &...&0.0610909 \\
\cline{2-6}
\end{tabular}
\end{table}
\end{document}

\centering, not thecenterenvironment, which adds unwanted vertical spacing. – Bernard Feb 26 '17 at 11:13IEEEtran, which is where this will end up. (+1 for your answer, but it is inappropriate, in my view, for sober scholarly work.) – jon Feb 26 '17 at 15:08IEEEtranpaper. That said, I think the first table I propose is quite sober. I agree the second is more for a book, although changing the rules colour to the same gray as the background is acceptable. – Bernard Feb 26 '17 at 15:20