I would definitely go the second way with $\displaystyle: I find that it looks strange to have something centered after a bullet. To get the vertical alignment correct, just add [t] after \begin{aligned}, where "t" means "top". (See also the top of page 8 of the User's guide for the amsmath Package.)
Here is a way to do it using references, and consequently at least two compilations. It requires using labelwidth rather than widest. First, some background...
The obvious motivation behind itemize and description providing solutions to this without problem is because the former (itemize) uses a single-element (symbol) to denote list items, all of which have the same width (by default). Overriding this default to anything other than a single-element (symbol) will have misaligned items, or item labels protruding margin lines:
\begin{itemize}
\item An item
\item[qwerty] An item
\item An item
\end{itemize}
This is due to the left-aligned nature of the itemized bullets. Another motivation is because lists are typeset "on the fly" and even allow for page breaks between items. So, in order to know the widest item at the start, the entire enumerate environment body would have to be read in memory/parsed before determining the widest, and then typeset - not the case by default. The latter (description) works because it uses a right-aligned approach to typesetting the itemized bullets and has a paragraph-style typesetting of the label and item contents by default.
The solution using enumerate requires setting the following enumitem options for a list:
labelindent=0pt: to have a flush left margin;labelwidth=\widthof{\ref{last-item}}: this requires the calc package that measures the width of \ref{last-item} where last-item is a label set for the last item in your list;label=\arabic*.: labels will be arabic, followed by a period .;itemindent=1em: separation/space between label and item; andleftmargin=!: automatically calculate what the leftmargin should be based on the provided parameters above (this is new to version 3.0 of enumitem).
Here is a minimal example highlighting the result:
\documentclass{article}
\usepackage{enumitem}% http://ctan.org/pkg/enumitem
\usepackage{calc}% http://ctan.org/pkg/calc
\begin{document}
\noindent Just enough text to make the line break so we get to see a second line.
\begin{enumerate}[labelindent=0pt,labelwidth=\widthof{\ref{last-item}},label=\arabic*.,itemindent=1em,leftmargin=!]
\item An item
\item An item
\item An item \addtocounter{enumi}{9997}
\item An item \label{last-item}
\end{enumerate}
Just enough text to make the line break so we get to see a second line.
\end{document}
Best Answer
From the comment of prettygully: The default is to have equations centered. use can use
\documentclass[fleqn]{article}to have all equations start a given distance from the left margin. this distance can be modified using e.g.\setlength{\mathindent}{2cm}