That depends on what you mean buy best? Wifi connectivity, power management, ease of use, etc?
I don't own one with built-in wifi but I do know that the Toshiba e740 and the iPAQ 5450 aren't the best at power management. I believe that 5450 has better software and may have better range due to the external antenna compared to the internal one in the e740. But as you can see there is a lot to consider with an all-in-one out-of-the-box solution.
Remember compromises will be made with any wifi solution you purchase. For example, my Socket Wifi card has great power management and good range but at a cost of the card and the sleeve.
Making use of mobile tech like no other.