I like Template Toolkit, but that's probably because it's what I use the most. Here's what I like about TT.

• Default syntax stands out from HTML (however, I believe you can change the default HTML::Template syntax if you want.)
• You can call methods on objects passed to your templates. Some people abhor this, but I like to do it for Class::DBI and DateTime objects.
• TT has built in methods for iterating over hashes and arrays, whereas with HT you have to coerce things into an AoH to iterate. TT even allows recursion.
• It's easy to install new VMethods in TT for doing things to arrays and hashes (and scalars.) This can be a real time-saver if you don't want to do these things in your Perl code.
• TT has a lot of plugins available, for handling everything from LaTeX to DOM stuff within your templates.
• You can embed Perl in your templates. This has never seemed like a good idea to me, but it's there if you want it.

You might also want to read a bit about HTML::Mason, which lets you embed Perl in HTML, PHP-style.