By the power of truth, I, while living, have conquered the universe.
Mythic Map Pack I was the best overall pack, IMO. Sandbox alone was a pivotal upgrade to Forge from Foundry and I remember fanboying over all the new elements it included when Bungie revealed them in the Weekly Update. I wasn't that big of a fan of Assembly, but Orbital was excellent.
MMP II was also really good. Citadel was ok, but seeing Midship return was a Halo 2 fan's wet dream and Longshore was a fun (yet heavily underutilised) BTB map.
That said, I loved all the maps in the Legendary Map Pack (yup, even Ghost Town). Blackout, like Heretic, was a remake we'd all been gagging for and Avalanche is still a map I'll never turn down for BTB CTF to this day.