Awhile back a discussion was started on if you had to choose only 5 weapons to equip your army with what would they be.
Not surprisingly the God tier weapons that ruled almost universally in the lists was the medium laser and Gauss Rifle.
So I was curious as to see what people considered to be the most well rounded weapons.
My choice for most well-rounded: Medium laser, LRM launchers, and PPCs. Between those 3 weapons there is little you cannot blow up on the battlefield. MLs (most forms) are efficient secondary weapons, PPCs (of all stripes) are workhorse heavy, ranged weapons, and LRMs' ammunition flexibility and indirect fire capability makes them, well, flexible. Can they destroy 'Mechs? Yep. Tanks? Yep. Infantry? Yep. Aircraft? Yep. Buildings? Yep. Short-range targets? Yep. Long range targets? Yep (PPCs and LRMs). Indirect fire? Yep (LRMs). Non-lethal and supporting munitions? Yep (LRMs). They're not the most effective weapons for all those tasks, but they can accomplish them, and the weapons that are better at one of those tasks tend to suffer in other roles (e.g., MGs: great infantry killers, not so good in the other roles.)
Of those, you only listed LRMs, so I voted "LRM 15" and "see below."
Note that the MLs, LRMs, and PPCs are my preference amongst well-rounded weapons. There are other fun combinations. The Gauss Rifle you mentioned is laudably flexible, too - there ain't much it can't blow up, though it's not to my taste.
And MLs, LRMs, and PPCs are not the most perfect
What weapon offers the best options for engaging different unit types from fighters to infantry and was the most effective at achieving those results while not making your mech loadout gimp?
You can get the job done with LRMs, MLs, and PPCs. There are better weapons for killing infantry, like MGs, but the trio of workhorses will do. LRMs with specialty ammo can be murderous, especially if they can use exploit their range against the infantry.
If I had to pick a single specialist weapon for anti-infantry work, it'd be the MG. Light, compact, lots of ammo, very effective. In fact, when picking 5 weapons for my military I'd think long and hard about making the MG one of them. (And on that note I'd probably pick an artillery piece to be the 5th.)