Short answer is - anyone you plan on using should have full masteries.

This is especially critical for CB champs, since you'll want leech or lifesteal with Warmaster or Giantslayer to provide your team with full healing.

anyone you use on Clanboss has priority for masteries, in this case I guess its Frozen Banshee

Masteries are really important for Clan Boss

So Frozen Banshee will benefit definately. 

But its a long way to gain masteries for all champs I use. So I need to prioritize and that was my question 😀

And the answer to your question should basically be determined by which ones you think you'll be using for the longest amount of time and/or which are the most important to your progression. That's hard for anyone else to answer.

From the ones you listed, Doompriest and Achak probably have the longest lifespan, and Frozen Banshee for CB. I haven't played with Bystophus or Tomb Lord so can't comment on them.

as I said Prioritize the champs you use on clanboss

beyond that:

get Doompriest atleast the first ones which improve its healing, that should be quick . full masteries is not a priority for Doompriest

I think Achak would benefit  most from full Masteries, after your clanboss champs

it also hugely depends where you use them:

my generalized priority list:

clanboss

arena

Dungeonbosses/Dt bosses

dps

debuffers

tanks

support