From least worthless to most worthless
Medivac - Medics are better healers and Hercules are better dropships
Reaper - Only useful in 3 missions
Raven - Science Vessel is better, sorry Raven :(
Goliath - Viking or MM is always better
Firebat - Some use in the begining, but quickly becomes obsolete
Vulture - Only good in it's own mission
Helion - same deal with vultures, although reapers are better
Diamondback - Too expensive and only good in it's own mission, where siege tanks are better
Predator - did anyone actually chosse this?
Wraith - fail unit is fail, not even good in its own mission
^^^ This. I'd say medivac is not really a useless unit at all though. Sure you listed 9 units more useless, but don't even bother mentioning the medivac. Medivacs are good because they are cheap for moving smaller forces, have more health than medics, and don't get attacked as much as medics. Hercules is great, but I didn't use it much at all (1 mission) because it's expensive and takes fusion core.
I also disagree with goliath just because of it's range compared to M&M, larger health, and excellent mineral to gas ratio (3:1) . It's 3 supply and increased 50 mineral cost from SC1 is unfortunate, but don't really matter much. Attack air and ground at the same time is pretty good too. Vikings are faster, better AA units, and about the same on the ground, but their transition period really sucks, especially when dealing with both ground units and air units.
Getting gols makes it so that you don't need to get marines at all, really. While you could say the same with vikings, they cost more gas, and have the annoying transition period.
I rank diamondback near the top of that list too. When I first saw it I was thinking "WTF awesome unit" cause it had a +1 range upgrade. 20+20 damage at 7 range attack while moving? sweet. Too bad it turned out to shoot too slow, and move too slow for either of those things to be too great, while it also costed a whole lot (GAS!)
Never tried predator, but looked like crap. Same with the wraith.
Also, I'd say just because science vessel is better, doesn't mean raven is a bad or useless unit. Both raven and science vessel are good units