As seen from the answers the OP question is a bit too open ended.
I'd say the standard answer is "it depends on the situation".
Terrain, Mission, Missile User, Allies, Targets, # of Tons of Ammo, all that comes into play when picking my load.
For Example....
Fast Mover w/ ATM and 3 tons........ I might go ahead & use a ton of Standard since I'm fast enough to dictate range & hit those limited sweet spots. I'd also use all 3 on some monster w/ 6+ tons of ammo. On something slower w/ only 2 tons. Standard is the one I skip.
The MML is even more situational based on allies.
Got TAG, take some Semi-Guided.
Got NARC launchers, load up on both LRM/SRM Narc ammo.
A specialist might focus on only 1 range but your typical siege engine "MML Carrier" will probably want plenty of LRM with at least a ton or 2 of SRM for dealing with backstabbers.
I'd say over all I try to have plenty of variety but w/ a bit of focus to it.
Say 50% ammo for a primary function w/ the other 50% split into supporting supporting areas.
So for 4 tons for the "MML Carrier" example, I might go 2 of SG-LRM to support the 3 units w/ TAG in the company, another standard LRM ton or perhaps Thunder LRM, and finally a ton of SRM for close encounters.