So much of it comes down to what the unit already looks like. I've played generalists and specialists, units that are very people centric, and units that are very by the numbers with a "life is cheap, mechs are not" mind set.
I tend to gravitate to specialist units, so I'd try and get something that makes me more of what I already am. So maybe that's some more XL engines if I'm a unit that focuses on speed. Maybe that's some BA if I'm a unit that uses infantry in combat (or, in one of my more favored units, scout BA to facilitate that aspect of my forces).
Truth be told, though, I'm usually pretty conservative about advanced tech, since I fear being able to replace or repair it in the field. I'd be reluctant to have a lance of Plasma Vindicators (great mechs) and then have my key medium trooper lance out of action if I can't get parts for Plasmas because I'm under contract to someone else.
That makes ammo the attractive choice for me, since its designed to be expended anyway and it goes with the weapons you already have. Fancy LRM ammo is a good go to, especially Thunder Augmented or semi-guided. If I'm saddled with a lot of 3025 mechs then maybe fancy AC ammo, but I might just pick a stockpile of Ultra or LB cannons in that case.
Though, perhaps a lame answer, if I've got a lot of old mechs, I'd just pick something really simple like double heat sinks. They're probably the biggest game changer out there, so if I can get DHS for a company or battalion or whatever of my old intro tech mechs, I can just jam enough old tech guns on to make my force a lot more potent.
