In a nutshell

[C2]

Ham radio.

I want to tell folks about the merits. I understand them to an extent.

What is the reliability of communication, say between the best mobile to mobile, mobile to base...at it's best configuration, and what would that be? Can it be a practical and reliable communication mobile to base?

 

[vkrules]

Reliable practical radio communication is basically line of site . That's the short answer that fits your criteria.

 

[Ham Man]

2X what VKRULES says.

As an example, a cell phone is a mobile device, while a tower is a fixed base station.
In the days of 2G coverage, it was very common to be in the shadow of the tower, could see the light blinking on the tower, and yet have no coverage.

The array of antenna's around the top of the tower is called the candelabra.
The purpose of the candelabra is to receive your signal, of which the tower can steer the antenna by putting multiple antenna's in or out of phase.
This is done both physically and electrically - it is called Beam Tilt.
This is done so it's microwave signal goes outward and downward.
When you get too close to the tower, the antenna at times can no longer see your signal, so you have a drop out. At least with that technology.

This is the reason why I know that it was impossible that anyone on flight 93 that was shot down on 9/11 could call their loved ones from the plane.
The antenna's do not look outward, and even if they did look outward, when you are at 30,000. feet, how many towers can you see. The system was designed for someone on the ground to talk to the tower. When you raise your elevation, your horizon changes and the system was designed to lock with the tower that had the strongest signal. Now all of a sudden you have 15 towers all trying to lock on your signal while you are traveling 400 mph and the system at that time was never designed to handshake from one tower to the next every 20 seconds.

With amateur radio, it is all about location... Location, location, location means everything. If your home base is down in a valley and you are down in another valley 10 miles away, there is very little hope for someone with a 50 watt 2 meter radio to talk to another 50 watt station. Put either station up on a mountaintop and you can hilltop as far as the eye can see and then some..

It would also depend upon the time of the day, the time of the season, the atmospheric conditions, the solar conditions and the frequency..
80 meters at dusk or dawn, you could probably talk 100 miles with no problems.
10 meters - a little more like 15 to 20 miles in the mountains and maybe 30 - 40 on flat land.

 

[RatsoW8]

A lot of assumptions being made in the replies without many details fro the OP.

 

[Moleculo]

Can it be a practical and reliable communication mobile to base?

Yes. Under all conditions and distances? To a practical point, but not worldwide (unless you factor in internet linked modes like D-Star, echolink, etc.)

"Can it" depends on a lot of factors, though. Need a bit more details on the conditions, distances, terrain, etc. that you're proposing.