Like many, I was using Manfrotto Video Monopods such as the 561BHDV, but later started working with the Sirui P-204S for the option of the sturdy 'locking base'. But recently Manfrotto has started offering the new XPRO Fluid Panning Base that includes a Vertical Locking feature.

The XPRO Video Monopods come in a few different flavors depending if you want Aluminum, Carbon Fiber, 4-Section, or a 5-Section and kits are available with or without a Video Head. These systems are fairly new and many websites haven't fully listed all the available options. The kit i've chosen is the MVMXPRO500US which is already available for purchase (found here).

Manfrotto MVMXPRO500US XPRO Video Monopod with Locking Base