My current preamble.tex I use for all my class notes and assignments.

preamble.tex

\documentclass[letterpaper]{article}

% Geometry
\usepackage[letterpaper, top=2.54cm, bottom=2.54cm, left=2.54cm, right=2.54cm]{geometry}

% Fancy Header
\usepackage{fancyhdr}
% Change header bottom bar thickness
\renewcommand{\headrulewidth}{0pt}

% Hyperlinks
% -option hidelinks disables red boxes around links
\usepackage[hidelinks]{hyperref}

% For pictures
\usepackage{graphicx}

% For special character support
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

% New center environment with no padding
\newenvironment{centernop}[1][\topsep]
  {\setlength{\topsep}{#1}\par\kern\topsep\centering}% \begin{centernop}[<len>]
  {\par\kern\topsep}% \end{centernop}

% Tikz
\usepackage{tikz}

% Phylogenetic Trees
% -http://cbr.jic.ac.uk/dicks/software/newicktree/
% -requires compiling with xelatex
%\usepackage{newicktree}

% Qtree
%\usepackage{qtree}

% Enumitem for formatting enumerated lists
\usepackage{enumitem}

% For hanging indents
\usepackage[notquote]{hanging}

% Listings
\usepackage{listings}
\lstset{
	tabsize=2,
	basicstyle=\ttfamily,
	aboveskip=0pt,
	belowskip=\baselineskip
}

% Courier
% -for \ttfamily to work in listings
\usepackage{courier}

% Paragraph box
\usepackage{pbox}

% Math symbols and other symbols
\usepackage{amsfonts}
\usepackage{amssymb}

% Colours
\usepackage{color}
\definecolor{grey}{gray}{0.4}

% Math tools
\usepackage{mathtools}
% \ceil{} and \floor{} functions
\DeclarePairedDelimiter\ceil{\lceil}{\rceil}
\DeclarePairedDelimiter\floor{\lfloor}{\rfloor}

% Tilde macro
\newcommand{\mytilde}{\raise.17ex\hbox{$\scriptstyle\mathtt{\sim}$}}

% Margin paragraph commands
\DeclareRobustCommand{\mymarginpar}[1]{%
 \mbox{}\marginpar[\raggedleft#1]{\raggedright#1}
 \vspace{-\baselineskip}
}

\def\mpconsider{\mymarginpar{\textit{Consider}}}
\def\mpclaim{\mymarginpar{\textbf{Claim}}}
\def\mpex{\mymarginpar{Ex.}}
\def\mpnote{\mymarginpar{\textit{Note}}}
\def\mpproof{\mymarginpar{\textbf{Proof}}}
\def\mpreadings{\mymarginpar{\textit{Readings}}}
\def\mpselftest{\mymarginpar{\textit{Self test}}}

% Quick Commands
\def\N{\mathbb{N}}
\def\R{\mathbb{R}}
\def\implies{\Rightarrow}
\def\and{\wedge}
\def\Or{\vee}