I like WAMP but I can’t find a portable version.
Which one is considered best?
The only one that I’m aware of is usbwebserver, I’ve not used myself so can’t say how good it is.
XAMPP has a portable version.
I am using zend server. I will try XAMPP which has portable version.
Thumbs up for XAMPP here. Used to run a USB version some years ago and it was great. Having said that - a dev area on a hosted server is pretty much the height of portability - you don’t need anything but your username and password! 
Is the portable XAMPP version the same as the normal installer?
Its just I downloaded the so called portable version but it still has an installer.
As far I know, there is unofficial XAMMP version without installer
I’ve had good luck in the past with the ‘Uniform Server’ but it’s a LAMP if I remember correctly. Very easy to use and can be easily modified to auto launch when the usb is plugged in. Even so far as automatically opening a file after the servers are fired up (in my case I auto launched a homepage).
I use MoWeS protable for example
I find usbwebserver great.
Just copy to a usb stick and give to the client or friend to test - it can run from the stick itself. Or zip up the folder and make available as a download from a web page (many people don’t like a big attachment by email) Once they acknowledge they have it working, I delete the download of course. Then on changing bits of code, I just send the relevant folder by email to copy over the existing one.