All of them
Since the text refers to maxing out a damage roll, then both Booming Blade and Divine Strike, (and Absorb Elements if a charge of that is available) damages dice should max out.
The idea behind this answer is that since all of the dice being rolled to apply thunder or lightning damage are linked to a single weapon attack, they are all one damage roll. (Similar mechanically to sneak attack dice being added to a single attack for a Rogue).
Reasoning behind this answer.
The question to resolve is whether all of the damage is rolled at once, or if there are separate rolls. With separate rolls, one could argue that one only applies it to one. But if you roll all dice at the same time, there is no reason not to apply "max" to the total roll for that one attack.
... Once on each of your turns when you hit a creature with a weapon Attack, you can cause the Attack to deal an extra 1d8 thunder damage to the target.
Did you make a weapon attack? Yes.
Part the booming blade cantrip is the making of a weapon attack. (You still have to hit if you desire Divine Strike damage).
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.
The d8 from Divine Strike is rolled in addition to the roll from booming blade. (And the weapon).
Channel Divinity: Destructive Wrath
... When you roll lightning or thunder damage, you can use your Channel Divinity to deal maximum damage, instead of rolling.
You made a weapon attack, you rolled various dice for thunder damage, max the entire roll. You have now used that instance of your channel divinity. If you have some stored lighting from Absorb Elements, the "lightning or thunder" text allows that to be added to the damage on that attack, and maxed as an eligible damage type for that use of channel divinity for that one weapon attack.
I can see a DM calling them out as "separate rolls" but at our tables, we tend to roll all of the damage at once to keep play moving.
This gets a little tricky with Booming Blade in particular, since the damage from the "when the target moves" feature typically happens at a later time when the target you hit moves ... which could be during its turn or even during a subsequent round.
The simplest way to deal with this is to max that damage once the
targer moves because it is explicitly linked to the particular
attack that used the Channel Divinity class feature (which is a
short rest resource, and thus not spammable round after round).
With that said, @Seidr makes the point that a ruling for a separate,
and not maxed, movement-boom-damage roll comes from deeper in that
tweet stream, if you assume that "other processes" is what that
Separate damage rolls are delivered by separate attack rolls, saving throws, or other processes. Extra/bonus/additional damage is called extra/bonus/additional damage.
@RubiksMoose Replying to @JeremyECrawford
how do you distinguish between second damage roll and extra damage?
While I give preference to that class feature since it is a limited use resource (my "specific over general" for this case) it would be worth discussing this element of the question with your DM to see what makes the most sense at your table if you expect to use this combo in play. The other approach makes sense if you feel that the intent of "other processes" includes that movement.