The most contribution is the Research Points, which can only be paid in time... (by claiming daily)...
Those are the most valuable over time...
Parallel to that, is donating PART/SHARD to the guild for Guild upgrades... but those are quite costly... and if whenever anyone leaves the guild, 50% of that is retained to the user that donated. The guild looses it all.
Keeping the maximum players of a guild claiming daily, is probably the best strategy in my view, for any guilds. Researched stuff is super valuable long term.
Thanks for the explanation :-)