Most worthy mk2 builds?.

I disagree. There are "burst" setups, or glass cannons, like DB Griff and Thunder Carnage. For those builds your statement is true, because if they are on the receiving end of a high-powered build (PDB Griff etc.) they're dead. They are built to quickly defeat an enemy while taking little or no damage themselves. Adding 20% HP to those builds will add little to their survivability.

On the other hand, we have builds that will take quite a lot of shots, like pretty much any midrange or sniper builds or tanks (Leo, Raijin, also Lancelot). For midrange (I'll focus on that because sniping works similar), they will often receive some damage from other midrangers and snipers. And we're talking about a lot of hits here, my favorite midrangers (Gekko Zeus or Tempest Molot Natasha) can take about 4 hits from a Zeus Fury and still live. Tanks of course are built around being able to absorb some damage. Adding 20% HP there will indeed increase the efficienc of the build a lot. The same holds for bots that have a built-in Ancile or physical shield, because making a robot Mk2 will increase both the HP of the bot and the shield. 

When to upgrade? The decision is not different from deciding whether to level up weapons or robot first. Apart from practical considerations (you don't want an upgrade to finish when you're not able to follow up), ask what does the most for your robot. Keep in mind that a robot level adds usually 6% HP (some robots, including Natasha, Fury, Doc, Patton and Golem, grow at 8% per level) while weapon DPS grows by 10% per level. Assuming that we can use HP*DPS is decent approximation of the robot value (for midrangers and tanks, not for glass cannons), you can simply calculate what is most efficient. Take for example my Facebook Gameroom Tempest Punisher Natasha, which was 10/8 a short time ago. Adding 10% more DPS (upgrading Tempests Punishers to level 9) takes exactly 150 hours and 18 million. Adding 8% HP takes 31 hours and 7 million. In fact, adding 16% HP (level the Natty up to 12) takes only 64 hours and 16 million. So I obviously went for the level 12 upgrade first before taking the weapons to level 9.

Now, upgrading a level 12 robot to Mk2 has the side effect that it won't be available for a while (or rather weaker until you reach level 9 or so). But aside from that, it's just a 20% increase for 500 gold and whatever it costs to take the robot from level 1 to level 12 again. In the case of that Natty, it's about 9 days and 27 million silver. So in the example above I would increase weapons to 9 and probably to 10 before I consider taking the Natty to Mk2.

Most other robots are much more expensive, though. A Lancelot takes 20 days and 55 million for a full Mk2 upgrade. Say you have a 12/11 Thorkalot, it takes 13 days and 72 million to upgrade the weapons to 12. So, 20% more HP to bot and shield for 20/55 or 10% more DPS for 13/72? Going Mk2 wins, hands down. A 12/10 Lancelot is more tricky, as weapon upgrade to 11 takes only 10 days and 44 million. I'd still favor the Mk2 upgrade in this situation.

Some WSP robots are rather cheap and take long. So if you are silver constrained and like a Carnage, Fujin or Raijin, you should focus on the robot. Say an Orkan Fujin at 12/10: Getting the robot to Mk2 (20% more HP and shield), you''ll need 27 days and 41 million. Getting the Orkans to level 12 (20% more power), you'll need 43 days but a whopping 168 million silver.

Bottom line: Most robots are best developed by getting them to 12/10, then upgrading the robot to Mk2 level 12, then weapons to (Mk1) 12.

See also my hangar development guide.