It depends on personal preference. Outlook obviously works well with Windows 7, but free options like Thunderbird or Windows Live Mail are perfectly adequate. Personally, I just use my Gmail web interface.
Windows Live Email (download here) is a good choice. Includes a good contact list and calendar.
Also handles newsgroups and news feeds if you use those type of tasks.
Live Essentials download also includes Live Messenger, Photo Gallery, and a few other tools, but you can select just the programs you actually want.
You can setup your ISP's email account into Live Mail and also integrate a free Live Mail account or free Gmail account as well and manage them all from Live Mail.