LRMs being an IDF weapon really cover a ton of bases. Without resorting to advanced rules, I think every force should have some LRMs in it if its a 'take all comers' style unit.
SRMs likewise offer so many special ammo types that some SRM units are pretty much mandatory (without advanced equipment). Infernos alone versus heavy battle armor has been a savior for me.
MMLs combine both of these things, but we dont have that many MML units, and the MML doesnt exist in all eras. Still, the MML longbow variants are monstrously efficient since they can do so much with their flexible ammo loadouts.
Some form of Flak with range is always handy, and like stated the LB10x is a pretty great all around gun and source of flak. AC10s with advanced ammo in later eras are also really handy, with a combo of precision/flak/flechette letting you take on a lot of different unit types. The rifleman 3Cr and hammerhands both pack dual ac10s designed for the role, but there is a lot of quad LAC5 units that also do the same. I also like the dual LB10x blackjack omni variant, or the dual lb5x Gargoyle Prime.
Pulse weapons, of all the energy weapons, make the most sense to me. Yes you have less range, but Pulse accuracy is just always worth it. You have LRMs already as the number 1 long range versatile attack, so Id rather round out a 'cover all bases' unit with 2-3 medium pulse lasers then an ER large laser/PPC. Cause when the target is something hard to hit, only pulse lasers cover that 'base', though precision ACs are almost as good if you have a few already.