Absolute favourite was sedimentology, especially for long-running servers. Very old though.
Dimensional Doors is a lot of fun too... but really my favourite mods are tiny and cosmetic:
Nyx: useless but pretty meteors
Owls: useless but pretty
Serene Seasons
Stiched Snow: Snow will slowly pile up
Spice of life: makes you use multiple food sources
Void fog
butterflies: useless but pretty
hardcore darkness: removes the minimum light that's always present
DesirePaths: Wears down grass as its walked on to create naturally formed dirt paths