Military power: Zann had many vehicles and a pretty powerful fleet, with a good amount of it being stealth. GO-TO had no real fleet that we know of (except for his Yacht). GO-TO had pretty much mercenaries, thugs, and HK-50s for ground forces, but Zann had trained soldiers, tanks, and the droidekas.
Didn't G0-T0 effectively have an unlimited supply of HK-50s though... HK-50s probably outclassed droidekas because they actually had some intelligence, tanks can be destroyed, and as to soldiers they wouldn't have a consistent level of skill and the HK-50s are more easily replaceable.
The other flaw in the argument is that Zann would still have his droidekas... G0-T0 was an expert at seizing control of droids. G0-T0 could easily walk off with a substancial amount of Zann's forces, even have them turn on Zann at the drop of a hat.
Stealth: They're both good at hiding, but they both were caught eventually. Zann really didn't have stealth when he was hiding out. but he was found by the hutt cartel. GO-TO had to be baited, and if it weren't for that, he probably would never have been found in the first place. Zann did have stealth ships, but only his personal ships and 1 other type of ship. GO-TOs yacht I think was pretty big and could easily stand up to just the stealth aspects on Zann's fleet.
G0-T0 would likely have an easier time locating Zann, than Zann would have locating G0-T0, the crime boss Goto never existed, but Zann wouldn't know that. Plus G0-T0 can easily reprogram large numbers of droids, meaning G0-T0 could easily have droid spies in Zann's organization without Zann realizing it.
Allies: Hanharr has holy **** grade strength, and Urai has cloaking fields, with a limited amount of force-sensitivity. Urai's force-sensitivity would probably have no effect whatsoever. If it ever came to a 1 vs 1, Hanharr wins hands down. Urai would have to strike fast and hard when Hanharr doesn't expect it to even have a chance to take him.
Interesting take, but I'm going to consider this brawl to be irrelevent in the grand scheme of the overall war.
For the leaders themselves, Zann probably would win if it came to a 1 vs 1, he could easily use the cloaking field to confuse and disorient GO-TO (if droids can get confused). And just gun him down slowly.
Problem with that argument is that G0-T0 has a built in stealth field generator, the most likely one to win that kind of a fight would be Zann, but it wouldn't be a landslide victory and could just as easily end up with G0-T0 winning.
In the end, GO-TOs forces probably won't be able to stand at all against a direct attack from Zann, but they'd probably find and be able to try to assassinate Zann before they got completely decimated. Zann wins because again, his military would prove too much for GO-TO, and he's used to operating more in the shadows than a direct assault, bribing regular people and some exceptional fighters (trandoshans, Gand, etc.), but against well trained and well equipped soldiers, he wouldn't last. Even if his Yacht was never found, it would still leave GO-TO stranded and unable to do anything without actually leaving his Yacht....
Considering G0-T0 can easily hijack part of Zann's armed forces right off the bat, it's safe to assume that Zann will have a sudden and rather bloody decrease in manpower as the droidekas turn on his soldiers.
Additionally, the fleet Zann has is relatively worthless because G0-T0's base of operations is completely mobile and cloaked. Then you add in the unlimited HK-50's (which are also walking explosive devices), and I don't see Zann continuing to enjoy his large army.
I find that to be extremely unlikely.
Zann will be hunting for "Goto" whom doesn't even exist, G0-T0 made him up, so we have Zann trying to hunt down an individual that doesn't exist, never suspecting he's battling against a droid.
Unless Zann is very lucky, I don't see how G0-T0 can lose this fight short of extremely bad luck.