There are two font packages I like for great readability (letters are clearly distinguishable, and math fonts stands out from text since it is heavier a bit). Unfortunately, they are buggy.
The first one, the one which I think gives the most readable math fonts is Boisik. I love its curly brace: very distinguishable from parentheses, brackets. Unfortunately, it has a problem with some small size fonts: it doesn't seem to have some of them in the 11pt and 12pt options to the class file, like article. Here is a small sample file
\documentclass[11pt]{article}
\usepackage{boisik}
\begin{document}
$$
T^{ U_{abcdefghijklmnopqrstuvwxyz0123456789}} \quad a \alpha
$$
\end{document}
When you compile (I have TeX Live 2012) the above, you won't get i,j,m,n,u. You get the same problem with the 12pt option, but no problem with 10pt, that is, without option. The above file also exhibits another problem (which seems to be common to other font packages as well), that the a and \alpha characters look almost the same.
Another package I have problem with is gfsneohellenic. For me, it has the best textfonts. But I cannot use the package in my work since in math mode the characters a and \alpha are exactly the same. Another problem with the package is that in subscripts it crams binary symbols with their surroundings. In regular environment, binaries seem to be fine. Here is a sample file
\documentclass[12pt]{article}
\usepackage[default]{gfsneohellenic}
\begin{document}
$$
a = \alpha\qquad \sum_{n \le N}\qquad n \le N.
$$
\end{document}

$$ .. $$should not be used with LaTeX, see Why is\[ … \]preferable to$$? – Torbjørn T. Apr 20 '13 at 15:10