It's a difficult question, as there really isn't one correct answer.
Technically, you could get away with a single "big weapon" (or even multiple "big weapons"), but it would depend on the weapon you selected. The preference would be something with no Minimum Range, or at least not a very large minimum range. If, for example, you selected PPC or Gauss, then you could probably live within the 2-3 hex Min Range issue they face. OTOH, if you picked LRM, then that 6-hex Min range is looking very limited, unless you're staying pretty far back.
To be fair, though, it's only been since Alpha Strike came out that CGL really focused more on the idea of actual "lance formations". I don't mean the "3 Mediums and 1 Heavy = Medium Lance" type, I mean the "lance role is determined by the role of the 'Mechs in the lance" type. Despite so many fluff descriptions indicating that most 'Mechs were designed with something specific in mind, most of the time the scenarios or even the various RATs just gave you random 'Mechs to fill out your lance. A Rifleman or Longbow is not really meant for the "line of battle", but all too often a bad RAT role might end up with your long-range-specific 'Mechs paired up with a couple of short-rangers...or you end up with a lance where you only have a bunch of short range units.
I would suggest that if you're going to design a custom-build around the idea, consider the old standards when it comes to real-world tank & ship designs (or at least through WWII for the latter). All of those designs had to balance speed, armor, and weaponry...especially the ships that were designed & built under the pre-WWII Washington Treaty. If you're going to use 1 or 2 long-range weapons as your only weapons, you're either planning for it to be strictly a long-range fire support platform...or you make sure it goes as fast as possible & has max armor protection for its tonnage. And I mean "as fast as possible". Don't stick to 5/8 or 5/8/5 for a 55-ton design; if you're going "1 big gun" on it, shoot for 6/9/6 (or faster if you can swing an XL engine & Endo Steel). If you've got a 100-ton design, pay the cost to make sure it goes 4/6 instead of 3/5, & consider putting all 4 jump jets on it. You can even consider some "secondary" weapons, but I would limit it to maybe 2 MLs (or SLs/ESLs) at most.