1. The Guillotine, in any version. It's a great jump capable, competitive low cost heavy raider and brawler that works well in any heavy lance. Even the 3025 downgrade is well done. Low heat, excellent heat sink capacity, and it really only has 2 bad variants the WB 2 and 3 (not enough heat sinks or LRM-15 ammo.) People never remember it when they're picking out designs, but the 3N, 5M and 8Ds are staples of any Comstar, Marik or Davion unit I use.
2. The Ostroc-This is the bread and butter of low cost 5/8 heavy brawler lance. You get two larges, two medos, and an SRM 4 rack, all torso mounted, while the arms act as damage absorbers. The Ostroc moves fast enough to be a command unit for the 55 ton trio, works well in groups, and looks great as well.
The 2D is a simple DHS/ER LL upgrade which is great for mercs, Periphery powers, or any medium tier House unit. Later variants add more guns, jump jets, ferro-fibrous armor, and so on, but it and the Ostol are both Marik staples. Death eggs unite!
3. The Crab. The Crab is another lower BV cost energy brawler that is a great trooper. The Royal variant is another easily made swap for Jihad era Crabs, by simply adding DHS and IS ER LL. I still don't know why the resource poor Clans, like Blood Spirit or Fire Mandrill, didn't upgrade the Royal Crab and add it as their main second line mech. You have to blow it apart in order to kill it.
4. The Chameleon 7Z- This is another surprise unit everyone overlooks once it's out in 3058. For 1527 BV you get a 50 tonner that moves 6/9/6, 300 XL, Endo Steel, 12 DHS, max armor, 1 ER Large and 2 regular IS Large Lasers. Add this to any heavy scout or skirmisher lance, and you have a mobile energy battery for your needs. By the Early Republic, everyone has access to it, and I can see it working especially well for the Magistracy as a lancemate for Agroteras and Shadow Hawks.
5. The Yeoman. Yes, it looks like a 1980s Gobot. Yes it has no secondary guns, except for the Jihad era variant which swaps out the LRM-15s for 4 MML 5s, jump jets, an XL gyro, and more armor. But it does its job, which is to provide LRM support. A lance of Yeomen is a deadly sight when you have Narc or TAG equipped spotters for semi-guided LRMs. Add in proper screening forces, and the Yeoman will usually come back home.