erb, haml or slim: which one do you suggest? And why?

ERB is good mainly if you have a web designer that will work on plain HTML and does not know either haml or slim. This way they can write HTML and you can embed ruby logic with the proper tags.

If you work on both HTML and ruby logic, or your designer is ready to learn something new (like HAML) I'd go for HAML. It is a lot more ruby-friendly, reduces char count by much and a lot more readable than ERB.

For example (taken from official HAML site):

In ERB your view will look like this:

<div id="profile">
  <div class="left column">
    <div id="date"><%= print_date %></div>
    <div id="address"><%= current_user.address %></div>
  </div>
  <div class="right column">
    <div id="email"><%= current_user.email %></div>
    <div id="bio"><%= current_user.bio %></div>
  </div>
</div>

While in HAML it will look like this:

#profile
  .left.column
    #date= print_date
    #address= current_user.address
  .right.column
    #email= current_user.email
    #bio= current_user.bio

A lot cleaner!

As for the difference between HAML and SLIM - I never really worked with SLIM but I guess it is a matter of taste - take a look at both syntaxes and decide which looks better in your eyes. I don't think there is a definite winner between those two (HAML/SLIM).

Two big advantages of using slim over haml:

1. Slim is currently about eight times faster than haml.

2. Slim supports HTTP streaming, while HAML doesn't.

3. Slim has a more natural syntax: a href="foo.html"