Windows 7 e-mail.

I don't think you really mean to ask about an "email server."  Rather, you probably mean, "How should I best access my email on my Windows 7 computer."

However, there really is no "best" when it comes to accessing email.

The first decision you need to make is whether you want to access your email (both sending and receiving) using a web browser (e.g., Mozilla Firefox, Google Chrome, Microsoft Edge, etc.) or if you want to have a program -- an email "client" -- installed on your computer.   There are pros and cons for each method of accessing email; see some here: https://www.google.com/search?q=webmail+vs+email+client.  You actually can use both, although that can get a bit complicated.

By default, Gmail is a web-based emails service.  Almost all ISPs also have a web portal from which to access your email.

Microsoft has confused things by using the same name to mean two different things.  Outlook.com (which once was Hotmail) is a web-based email service.  Microsoft Outlook, which is a component of Microsoft Office, is an email client that is installed on your computer.  Probably most corporate email users will use Microsoft Outlook.

It all comes down to what works best for you. 

Do you like using Gmail?  If so, there's no reason to stop using it. 

Do you have Microsoft Outlook or do you use Outlook.com?  If you have Microsoft Outlook and like it, there's no reason to stop using it.  In fact, you can add your Gmail account to Microsoft Outlook and access it that way in addition (or instead of) using the Gmail web interface?

Personally, I use Mozilla Thunderbird (an email client installed on my computer), mainly because I've used it for years and am quite used to it.  On my Windows 7 system, I use Thunderbird to access several Gmail accounts, a Hotmail (Outlook.com) account, my college alumni mail account, and my ISP's email account.  I also use it to subscribe to the NOAA Weather RSS feed from the National Hurricane Center.  I used to use Thunderbird to access news groups, but hardly anyone uses news groups anymore.

To add to Lem's good advice and explanation:

Consider Windows Live Mail.  It is likely already on your computer and you may not know it.  It is part of Windows Live Essentials.

WLM is really the grandson of Windows XP's extremely popular Outlook Express.

As Lem explained, this is not webmail.  It is a program that resides on your computer and leaves you with the storage and responsibility to manage your mail.  Your contacts and your email remain on your computer.  Just like Thunderbird.

If you do not have Windows Live Essentials and therefore WLM, you can still download it from:  

http://web.archive.org/web/20160423192125/http://wl.dlservice.microsoft.com/download/1/9/4/194B1919-A211-441F-BC0E-34F6B533B876/en-gb/wlsetup-all.exe