So we will divide this into Terran, Zerg and Protoss.
Rule
_ This unit you must be able to build more than 1
_ Building is not counted (so no Infested Bunker)
_ Only player’s controlled units.
Zerg from lowest to highest
Terran
Protoss
Best in terms of what? As a strategy or personal preference or cool points?
Spam-able Terran only marines come to mind. Zerg is clearly mecha zerglings. And for Protoss it’s definitely zealot/sentinel.
On a separate note, I’d like to point out your Terran and Protoss choices are basically abilities and not really units. Mengsk BC caps usually at 9 (15supply a pop) while DA is many but again not a damage dealer (so clearly spamming MC).
So if people are to follow this train of thought, it largely deviates from your “rules”. So my choices would be like “Stim” instead of marine, which sounds silly. Silly in the sense that your topic should be about “spamable ability” rather than “spamable unit”. After all, your BC and DA choices are expensive in most senses.
Anything with Zagara. >.>
I interpret your question as which unit could you win the most easily with if you could build a single type of unit.
Terrans
From worst to best
Swann’s Goliaths would be better of with SV but are good enough as is.
Raynor’s marines with drop mastery are pretty good even without medics if you feel inhumane.
H&H reapers.
Speaking of: Mengsk P3 troopers. Those are some great banelings and they even hit air.
Pro strat: Raynor P1 full BC.
Protoss
From worst to best
Vorazun VRs
Alarak P3 Destroyer (just be mindful of AOE spells 
)
Artanis Dragoons, great to spam.
Pro strat: full carriers/tempest with 3 forges, two cyber and researching everything before building units.
Zergs:
From worst to best
Kerrigan’s muta. Like the destroyers except they are even more vulnerable and go to their death much faster.
Abathur’s mutas (much more resilient)
Dehaka’s hydra
Stetman’s hydra
Kerrigan’s hydras squishy but nice.
Dehaka muta.
Stukov P1 diamondback (except on Void launch).
Pro strat?
I still feel 9 Mengsk’s BCs are best units. Can clear the wave with minimal damage taken. Deal with many situation. Kill props and unlikely to die.
Creeper hosts are strong too. Since it can attack from very far. It is also not likely to be killed and it can destroy any wave and fortification.
You can’t get a 200 supply army stronger than those 
Zagara’s units have to be re-filled again and again so it costs extra resources.
I am assuming combat units only not workers. Workers are easiest to spam.
Cheapest to spam and replace:
Terrans : Marines (Raynor) / troopers (Mengsk) / reapers (HH)
Protoss: Zealots not Sentinents and Legionaries
Zerg: Zerglings, Zagara suicide bombers
“Bang for your buck” spam, don’t need to replace often:
Terrans: Goliaths (Swann), Vikings (Raynor, HH), siege tanks (Raynor, Swann)
Protoss: Dragoons (Artanis), Adepts (Fenix)
Zergs: mutatisks (Kerrigan / Dehaka), Hyralisks (Kerrigan)
No elite units included.
ok i removed the spamable part since it makes people confused.
In that case, as “best” unit goes. I’d say…
Zerg - Brutalisk (for being cheap, accessible, versatile, mobile, and powerful).
Terran - Marine (still this one cuz cheap, mobile, high DPS, easily accessible).
Protoss - Fanatic or Shadow Guard (a toss up, these still obey your rules and are far stronger than anything else for their accessibility. I’d gone with Fanatics except their much more limited compare to SG in P3).
Assuming if these units were controlled from Amon’s side of things. Like if they were incorporated into Amon’s attack waves and stuff.
Terran: Mengsk Battlecruiser with it’s triple aoe yamato. Considering that it only needs to see one enemy unit then the other yamatos just seek out other targets means this is more difficult to deal with than most aoe in coop.
Zerg: Tyrannozor. Very strong antiair and they also reflect damage.
Protoss: Dark Archon. Both abilities are really good against players. Confusion can pretty much wipe a player’s army
Answers from the players’ side of things is far more complicated.
Going 3 by 3 list, they’re not the very best units, but goes high-up alongside them.
Terran
Mengsk Aigis guards: best marauder variant, very cheap in both cost and supply especially with P2.
Nova’s Liberators: one of the best siege units, arguably best for being an air unit.
Swann’s goliaths: great all-rounder units, with some science vessels it works agaisnt everything once you reach critical mass.
Zerg
Stetmann Mecha zerglings: not a surprise, it’s the best zergling variant, does a lot for the cost thanks to the barrier.
Abathur Ravagers: Weird isn’t it? But it’s a very good unit to use on P1 or other prestiges just for splash dmg, but then again, what Abathur unit is useless? Even roaches with biomass are good despite being his worst unit if you’re not going for brutalisks.
Zagara’s banelings: it’s Zagara’s butter for any army comp she uses, it’s the reason she can keep up in the late-game.
Protoss
Alarak Wrathwalkers: normally they’re already pretty good, a nice substitute for ascendants if you got bored of the same build, however P1 they go so much further, the dmg they do is real.
Karax Carriers: only thanks to Prestige 2, otherwise they are too much expensive to bother.
Zeratul ambushers: You can mass them and it’ll work agaisnt anything, it’s a surprise how some Zeratul players don’t really abuse the blinks.
If this is the rule then it’s hard to argue anything other than Abathur’s Brutalisks being the best Zerg unit. If we’re not counting those then I’d say Abathur’s Mutalisks being the best.
For Terran I gotta go with Raynor’s Marines. Cheap, easily replaceable, super strong with upgrades and masteries. They got it all. Close second might be Nova’s Marines.
Protoss is a lot tougher, I think overall I gotta give this to Artanis’ Archons. But that might just be because they’re my favorite.
This is for people who is curious to see Tyrannozor vs Brutalisk.
All units benefit from upgrades, research, mastery.
Calling shenanigans on tyran, Brutalisk would kite the crap out of it for a win. My money still on Brutalisk.
I would divide the best units as the most useful. and the most powerful.
Artanis’s Dragoon (Most Useful) - Cheap, versatile, durable, ranged (able to target sky and ground) and powerful. I think there is no question to this.
Alarak’s Full powered Ascendants (Most Powerful) - They are the most powerful spell casters in terms of sheer destructive force, but the problem is their energy costs. Karax’s carriers have far superior sustained attack though but Ascendents with their spells fare better against enemies.
103 stack Xel’naga Enforcer (Honorable Most Powerful)
226 (565 vs Armour) damage, 452 (1130 vs Armour) air damage, 103 shield defence, 104 armour defence. Unless you hit them with spell or yamato they are nigh unstoppable.
Kitings are for suckers who know they can’t win. lol
Stukov infested marines:
I give them my “UndeadStar could play in Hard with that” stamp of approval 
That’s funny. Approved.
Protoss: Ascendants. 1 full stack Ascendant + 1 Supplicant can kill a big red or big blue Hybrid all by itself.
Since Protoss is already covered with Ascendants and I can’t nominate Alarak’s Mothership since it’s a unique unit, I’ll go ahead with Terran.
Horner’s upgraded Battlecruisers deserve at least an honorable mention. They can warp anywhere on the map, deal a lot of damage, are very tanky, can even switch attack mode if needed, and they regenerate health out of combat, eliminating SCV maintenance. Amazing. Best Battlecruisers in Co-op. They remind me of the SC1 BCs, too bad we don’t have the real classic one in SC2.
Rank 3 Pride of Augustgrad wipes out waves with one Yamato, look shiny and increase the range of your ground units by 1. However, when Yamato’s on cooldown, Horner’s BCs are more impressive.
About worst units, here is what I think.