Which Z-Move has the highest base power?

When looking solely at the base power of certain Z-Moves the two strongest are Catastropika by Pikachu and Pulverizing pancake by Snorlax with a power of 210 each.

Note that only pokemon specific Z-Moves actually have a base power. The type specific ones (such as Breakneck Blitz) will scale off the chosen move. This means that a Breakneck Blitz used through Quick attack will be weaker (BP 100) than a Breakneck Blitz used through Mega Kick (BP 190).

The original attack will be chosen through the Z-Move overlay. As soon as you press the Z-Move button you'll see the overlay and if you have multiple compatible attacks you'll have the same Z-Move multiple times.
For example: If you have Quick Attack as the first entry on your attack list and Mega Kick as your 3rd you'll see 2 Breakneck Blitz on the 1st and 3rd position.

Another interesting attack in terms of strength is the Guardian of Alola move which deals damage equal to 75% of the opponents current health.

Inferno Overdrive when using V-create has 220 base power, making it the Z-move with the highest possible base power, as well as the second-strongest move after Explosion.