Well, the answer is it really does depend on what you are looking for. But one thing for sure all 3 of these webmail offers the basic email features such as : Compose Email, Forwarding, Junk Mail, Address Book, Trash, Folders, Marked as Read/Unread and many more.
SquirrelMail has all the basic functionality you would want from an email client,calendar, spell check, including strong MIME support, address books, and folder manipulation. It has simple interface where it basically list down all your emails. Recommended to those looking for basic read and reply email functions.
Roundcube offers much more nice, user-friendly and fairly intuitive interface for your emails as compared to squirellmail. It offers features such as markings, calendars,folders, full featured address book with groups and LDAP connectors, find-as-you-type address book integration as well as MIME support.
Horde, it is considered most fully featured email compared to the earlier two Squirrelmail & Roundcube and it has similar functions to Outlook .Horde webmail comes with features such as calendar, task list, notebook, address book, and more. The e-mail management system includes support for advanced filters, attachments, spell checking, and displays HTML e-mail well.
As a conclusion, your choice of which webmail to use really much depends on what you are comfortable with and what features or functions you are looking at. If you are looking for simplicity, go for SquirrelMail. If you are looking for appearance plus easy-to-use functions, go for Roundcube. Finally, if you are looking at email with more complete features , go for Horde.