DTs are good against Zerg period... Overseers are huge floating stalker targets. And no, Ultras are only good with proper Infestor support. Marines, Ghosts Roach masses, Stalkers masses, Immortals... so many things just rip through them and they are slow.
Play some Zerg or watch a ton of pro matches, then come back for Zerg unit evaluation.
I actually play random, have for quite some time. That being said I've used all the units, ultras tear apart people unless they went mass immortal(lol stalkers countering ultas, only in numbers of 2-4 maybe)
without upgrade it takes 4 swings(assuming terran has combat shield) for an ultra to take down a marine
marines do 7 dps, meaning it'd take 71 seconds for one marine to kill an ultra, or 71 marines in one second
stimmed a marine does 10.5 and takes 10 damage(meaning only three hits from an ultra) taking 47 seconds to kill an ultra with one marine (based on 0 armor ultralisk)
now, I don't feel like going through every upgrade I'm going to do max vs max upgrades on each
Marine max armor, 3, damage 9 max, 55 health max, stim takes 10
Ultralisk max armor 6, max damage 21, heath 500
So at max a marine only does 3 damage per shot
At max an Ultralisk does 18 damage per hit against a marine
anyway I'm in the unit tester
no upgrades on either unit, (did minerals cause marines cost 0 gas) and equal supply results in ultralisks destroying marines, 6 alive, out of 8 vs 48 marines
1/1 upgrades for both ultras win with 5 living
2/2 upgrades 5 living
3/3 7 living(with 4 at low health
max upgrades on marines and ultralisk 7 ultralisks remaining
so...ultralisks are nice
let's add in medivacs and maraduers
9 maraduers and 18 marines and 6 medivacs lose to 8 ultralisks...so..yea(can't find how to stim, that'd probably make it closer, but the ultras still had 6 full health at the end)
you're just plan wrong dude(eqaul minerals, almost eqaul gas)