The following post contains a discussion relating to the competitive M:TG standard format. For the vast majority of readers it will consequently be of no interest, so if you are among them, please don't waste your time.
1) Bloodbraid Elf -- Jund is built around her. Anyone who has played in the current standard environment has suffered the emasculation of a turn 4 bloodbraid into Blightning and the resulting 6 points of damage, two cards dropped, and a 3/2 on the board for a mere 4cc. Barring exceptionally poor play, BBE virtually always results in card advantage.
2) Baneslayer Angel -- As a veteran player, I can remember when Serra Angel was one of the game's best creatures. Yes, the same Serra Angel that is currently legal in the standard environment. Since it does not see play, you're forgiven for having to look up what it is. Baneslayer is an order of magnitude better. Her only drawback is that, despite being excessively overpowered, she is still a vanilla card, creating no card advantage and being a magnet for the opposing player's removal arsenal immediately upon hitting the board.
3) Lotus Cobra -- After a six-year hiatus, I've only been back in the game for several months, so I am unable to offer an assessment of value in anything but the standard environment. Consequently, I'm unsure if cobra is as stellar in extended as he is in standard. That said, with the near-ubiquity of sac lands in competitive standard play, cobra leads to absurd acceleration. Turn 3 Baneslayer is brutal.
4) Knight of the Reliquary -- The synergy with sac lands is obvious, but Tectonic Edge and the manlands brought in with Worldwake make her not only a potent kill condition for 3cc, but also an engine for control and support. If you're unfamiliar with Conflux, note that her activation allows a player to fetch any land, not just a basic.
5) Dauntless Escort -- Protection from mass removal with a body capable of throwing its weight around, too. Only recently have I begun to see boss naya and lightsaber running day of judgment, but in concert they provide a novablast wurm for 4cc.
6) Gatekeeper of Malakir -- When I played mono black HD/LD back in the early part of the previous decade, Diabolic Edict was rightly hailed as one of the best removal spells in the game, especially against control. Now edict comes with a 2/2 vampire attached to it. Shroud that!
7) Siege-Gang Commander -- Four guys for 5cc, each of whom can be thrown for two damage at instant speed (and all other goblins, too, if that is the deck's theme). SGC can take down a BSA and still leave his controller with a Mogg fanatic on steroids on the board.
8) Ranger of Eos -- A recurring theme among those making the list of top-tier creatures is that of card advantage. Ranger provides that, and the guys he finds are those who, despite their low casting costs, are actually better mid-game than they are early on, like Wild Nacatl and Scute Mob. Ranger hasn't had the monumental impact another certain 3/2 for 4cc has had, but he's still great.
9) Iona, Shield of Emeria -- If she hits the board against a mono-colored deck, the game's over. Naming white against bant or naya and black against jund similarly puts the opposing player out of commission. When esper control was viable, Rise from the Grave got around the hefty 9cc. Polymorph still does. With manlands inflating total land counts and the introduction of everflowing chalice, RWU and WU control are both able to simply hard-cast her as well.
10) Kor Firewalker -- Red deck wins has essentially been shut down by a single 2/2 for 2cc (although by splashing black or white, the decktype is still viable). He's also terrific against jund, especially builds that forego Putrid Leech.
Honorable mentions: Great Sable Stag; Sprouting Thrinax; Sphinx of Jwar Isle; Ob Nixilis, The Fallen; Goblin Ruinblaster; Putrid Leech; and Vampire Nighthawk.
A list based on competitive presence can be found here. It's not satisfying as a measure of individual utility, however, as jund continues to have the largest presence among deck tops and consequently its roster occupies all of the top slots. Putrid Leech is a worthy aggressive, fast drop, but it's not better than Baneslayer even though it's currently seeing more play than she is.