The acknowledge to this ask is easily a personal one. Everyone's likes, wants, and needs will be different. Now... How do you determine which is right for You?
In all honesty there is no 'best" smartphone, just the best one currently ready for your needs. For 5 dissimilar use cases, there will be 5 dissimilar "best" choices, so I can only guess as to your use case.
Is there a singular carrier you have to use?
Also, there are lots of dissimilar definitions of "smartphone". To me a smartphone means that I can load my own applications on it. It can get my e-mails, contacts, and appointments over air from a Ms change server. And that it has a means for essential text entry, for responding to e-mails.
There are some great phones that don't meet my definition (Iphone doesn't yet have an legal way to load your own applications, Sidekick doesn't let you load apps, Nokia N95 doesn't have a high volume text input method, etc...). There is lots of room for experienced users to disagree with me on this definition, so be warned.
The Windows mobile Os (current version is 6) is used in the Motorola Q series, the Blackjack I/Ii, Palm treo 750 and many of the Htc phones.
The Rim Os is used in all Blackberries.
There are other Os out there (Osx for the Iphone, Palm for the Treo 755/Centro, Symbian for some of the Nokia phones, etc...). But only the Ms Os has a universal way for syncing over air to a corporate change server. All the others want an added piece of middleware. Of those solutions, the Rim Blackberry business Server is by far the most popular. So if I have to guess for you, I'd limit myself to those two Os's.
The Rim Os is pretty easy to use, reliable, and straight forward. It does what it's intended to use well. That's why for non technical users, I roughly always recommend a Rim based phone. It may not have every bell and whistle but it's a workhorse for sending and receiving e-mail.
In Blackberries you basically have the option of a full size keypad (Blackberry 8800 or "Curve" style), or you have a streamlined keyboard (8100 or "Pearl" style). The Curves are much great if you have to write a lot of e-mail. The Pearl's are great if you need a smaller form-factor, and are reading more e-mail than you are writting.
In the windows Os there are many dissimilar shapes and sizes. I prefer units with a full keyboard like the Motorola Qm or the Blackjack Ii. Slider phones like the Wave, Wing, etc... You'd need to try some to pick the form-factor you like, but they all will accomplish similarly since they are based on the same Os. In normal the windows based phones can do more, but are more flaky and less reliable. It's not uncommon for some windows based phones to occasionally crash and the user needs to take off the battery to reboot the phone (insert your favorite anti-Microsoft joke here).
Once you've picked a carrier, Os, and form-factor, here's some final criteria to look at.....
A detachable battery is a must (another deal break with the iPhone). Is there an option for a higher capacity battery if you don't mind a bit more bulk.
How does it charge? A approved mini Usb port will make it way easy to fee in a variety of circumstances and save you money on chargers.
Does it sustain the latest/fastest wireless data for your carrier? Evdo Rev A for Sprint and Verizon, Hsdpa for At&T.
Does it have a detachable media slot (micro Sd, etc...).
Does it have an integrated Gps chip? Has the wireless carrier turned off that chip so you aren't allowed to use it? Is the chip the very sensitive (a good thing) Sirf Iii chip?
Do you need a camera? 2 Meg is a nice bump over 1.3 as it gives you good adequate resolution to easily take pictures of business cards and then have software turn the picture to touch data.
Do you need WiFi?
If I acknowledge all of the above for me, no phone exisits that has it all.
If I had to guess for you with no more information... I'd go with a Blackberry 8800.
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