Lemme leave my precise suggestions:
ze_FFVII_Mako_Reactor_v5_3
ZE_FFVII_Mako_Reactor_V6_B08
(if we have enough players so we can make it to ex2)
ze_mountain_escape_v5_zy
(we could play the map whilst admins are switching to a different bossfight/mode each round, like castle, flood, space etc.)
ze_pirates_port_royal_v5_4s2
(we can try to reach lvl 5 and beat the kraken, and to make it a little more challenging, maybe the zombies can get 10-15% more speed + more health; this way humans will have to defend and not doorhug, otherwise we will die)
ze_shroomforest_v4_5
ze_shroomforest2_v1
ze_shroomforest3_b6_2
(we could attempt an entire shroomforest marathon, so we can play the whole continuous story of all maps in one session)
ze_fapescape_v1_2
ze_fapescape_rote_v1_3f
(these maps are often underestimated and left out of consideration in here, but that's a shame because they're very well built maps with multiple stages, bosses and items, and actually require good teamwork in the extreme stages, especially rote)
ze_lotr_helms_deep_v5
ze_lotr_mines_of_moria_v6_3
ze_lotr_mount_doom_v4_2
ze_lotr_minas_tirith_v3_3
(these maps could be part of a lotr marathon, playing through the easier ones at first and ending it with minas; to make it a little more challenging, again zombies could have more hp and faster speed throughout the maps)
ze_ffvii_cosmo_canyon_v5fix
ze_ffxii_ridorana_cataract_t4
ze_ffxii_feywood_b3_1
ze_ffxii_westersand_v8zeta1
(these can make for a great ff marathon; perhaps on westersand we could enable insane mode on every stage for extra difficulty)
ze_minecraft_adventure_v1_2c
(it's a great map but we rarely have enough players to complete all stages)
Other worthy suggestions:
ze_tesv_skyrim_v4fix
ze_harry_potter_v2_1
ze_santassination_css2
ze_rooftop_madness_v1_9
ze_sandstorm_f
ze_tyranny_v5fix
ze_predator_ultimate_v3
