GMail, iCloud, or Yahoo. Any of these can keep your all your iPads in sync. All of them are free.
With GMail you need to
set it up as an Exchange account to sync contacts, but that's not all that hard. It is the most flexible when it comes to hardware choices, since it can be synced to just about any device or computer.
Yahoo is pretty good with all it offers to sync (since the 5.0 update). However, unless you pay for an account you are limited to browser access on computers. Of the three services mentioned, it also gets the most complaints for temporary server problems, especially on the mobile side.
iCloud is the most flexible and powerful between iOS devices. It's ability to sync with Windows is limited, but adequate, and of course it works well with Apple's OS X computer line. The main thing it offers is more flexibility to create, manage, and sync your stuff. It is will not work with hardly anything else. You do have access to your Calendar, Contacts, and Mail from a desktop web browser.
So my recommendations:
Yahoo: If you already have the account you might as well use it.
Google If your primary concern is the ability to get your stuff from anywhere from any device, whether you own it yet or not.
iCloud: If you're an iOS/Apple person, or just want to get the most flexibility from the built in apps.
Of course, there is no reason why you can't have and use all these accounts. I do.
Set up either the Yahoo or GMail accounts go to Settings > Mail, Contacts, Calendars > Add Account. Be sure to follow the
Exchange setup link for GMail.
For iCloud you must be using iOS 5. It has it's own set up in Settings > iCloud. Though you can set it up in the same place as the other account, you are bette off doing it here. No, I'm not sure why, but one user ended up with his set up in two places, and it cause a few problems.