Being the person that has amassed the most legitimate Gargantuas in history outside of havoc, I can tell you for a fact that Harriers/F117's are the best unit to convert for Gargantuas. The reasoning is fairly simple when it's explained:
1 Gargantua = £1,000,000
1 F117 = £137,000 (7.2 F117 = 1 Gargantua)
1 Harrier = £75,000 (13.3 Harrier = 1 Gargantua)
Now Scrap Bots get around 1:0.5 on each of these units - And fire before both in the close tick that they fire. Now from my work and research, the conversion rate on pure armour is around 72-76%.
So, in simple terms - ~70% of the Harriers/F117 Scrap Bots would kill, would be converted into Gargantua (as above calculations show how many of each would create one Gargantua).
There is no other mechanical unit in the entire game that gives anywhere near a better ratio of value/physical weakness than those two units. Absolutely nothing. I spent a whole round dedicated to converting and amassing the largest collection of Gargantuas ever seen (even cheaters have never gotten close to the number I managed to get).
Now Nanobots weren't the key unit in my army, although I always possessed between 15-30m at any one time at a rough guide, purely to use to counter non-LET based routes to eat and convert their flak (or any silly SO players that would dare stay against them).
If you're active enough to simply use Nanos/Scrappers/Gargs, then you are instantly terrifying. Example:
[close] 14,984,089 allied Scrap Bot attacked, killing 44,299,701 hostile staff.
[raised] The scrap bots quickly went through the remains, constructing 219,521 Gargantua.
[close] 1,398,691 allied Gargantua roared robotically and vaporised with laser eyes 340,740,546 hostile staff.
Died: 385,475,699 [£1,807,903,661,000] enemies dead.
[close] 1,231,953 allied Gargantua roared robotically and vaporised with laser eyes 23,542,057 hostile staff.
Died: 43,917,809 [£1,668,840,096,000] enemies dead.
[middle] 9,573,901 allied Nanobot proved resistance was futile to 31,870,537 hostile staff.
[raised] 1,056,569 new nanobots were assimilated.
[middle] 954,009 allied Gargantua roared robotically and vaporised with laser eyes 23,279,105 hostile staff.
Died: 55,149,642 [£745,048,925,200] enemies dead.
[close] 11,484,565 allied Nanobot proved resistance was futile to 688,831 hostile staff.
[raised] 327,812 new nanobots were assimilated.
[close] 957,457 allied Gargantua roared robotically and vaporised with laser eyes 17,035,687 hostile staff.
Died: 21,807,942 [£803,132,764,000] enemies dead.
ON PURE HARRIERS:
[close] 30,000,000 allied Scrap Bot attacked, killing 64,098,546 hostile staff.
[raised] The scrap bots quickly went through the remains, constructing 944,771 Gargantua.
Died: 64,098,546 [£1,140,667,853,600] enemies dead.
Converted: 944,771 [£944,771,000,000] enemies converted.
[middle] 23,815,200 allied Nanobot proved resistance was futile to 9,134,120 hostile staff.
[raised] 1,273,132 new nanobots were assimilated.
[middle] 2,086,925 allied Gargantua roared robotically and vaporised with laser eyes 10,783,814 hostile staff.
Died: 25,130,222 [£888,051,768,000] enemies dead.
[close] 27,221,423 allied Nanobot proved resistance was futile to 57,012,789 hostile staff.
[raised] 4,417,676 new nanobots were assimilated.
[close] 2,100,357 allied Gargantua roared robotically and vaporised with laser eyes 1,130,875 hostile staff.
Died: 176,993,651 [£1,406,871,013,900] enemies dead.
[middle] 10,624,232 allied Nanobot proved resistance was futile to 32,241,706 hostile staff.
[raised] 1,086,972 new nanobots were assimilated.
[middle] 987,623 allied Gargantua roared robotically and vaporised with laser eyes 21,649,019 hostile staff.
Died: 53,890,725 [£821,256,958,700] enemies dead.
[close] 11,625,154 allied Nanobot proved resistance was futile to 783,771 hostile staff.
[raised] 294,727 new nanobots were assimilated.
[close] 989,874 allied Gargantua roared robotically and vaporised with laser eyes 14,774,375 hostile staff.
Died: 21,049,111 [£883,138,939,900] enemies dead.