3.
Heartless Act. It's basically a
Murderblade except for when it isn't. That wouldn't be a problem in of itself, but the issue lies in what condition makes it not a hard kill effect. Counters on creatures is a very random stipulation. You might need to outright kill a creature with a counters on it, but won't be able to due to the random restriction.
2.
Ultimate Price. I like this card a lot. It's restriction also tends not to hit gold "signpost" creatures, which is a big plus for some formats. I would have this tied for 1., but it is a little bit worse than Go for the Throat.
1.
Go for the Throat. It's an efficient removal spell, not much more to say.
Card you didn't mention:
Cast Down.
Cast Down is awesome because it tends to miss a lot of engine pieces. Ever since WOTC started to support Commander as a primary format, a lot of the "cool" creature cards are now legendary.
Cast Down can deal with efficient threats like
Bonecrusher Giant and
Siege Rhino effectively, but spares several unique engine pieces such as
The Gitrog Monster and
Grumgully, the Generous.