Dark Ritual
 
It's interesting that fast mana is a lot less broken in severely restricted formats. For example, there's never been a serious push (as far as I know) to restrict Dark Ritual in 93/94. There are certain crazy things you can do with it, like a first-turn Mind Twist or Nether Void, but it's actually on par with things every color can do. Even colors that were historically weak in Vintage (white and green) have things that are perhaps lower-key but equally advantageous, like Land Tax and Gaea's Touch. It's true that Dark Ritual makes everything more powerful because they become effectively cheaper, at least in a vacuum. In actual Magic, just as in life, context is everything; there is no reason to avoid cards like this just because Wizards of the Coast avoids printing them in a highly-engineered format like Standard. Go ahead and cast your Sengir Vampires with it - that's what it's for.
 
Constructed: 5/5
Casual: 5/5
Limited: 4/5
Multiplayer: 4/5
EDH/Commander: 4/5

Dark Ritual (5/25)

Another simple card: turn 1 black mana into 3 mana. It enables turn 1 Hypnotic Specter/Vampire Nighthawk/Necropotence, it fuels a lot of black’s early aggressive starts, and it’s rarely ever a bad draw. It's maybe not what you want to topdeck in a game where you're out of gas, but there's a reason Dark Ritual shaped years of Standard and continues to have an impact, albeit lessened, in Legacy and Vintage.

Constructed: 4.5

Casual: 5

Limited: 5

Multiplayer: 3

Commander: 3.25