Deckbuilding Step 3: Winning Image
Yuri described this well enough in the anime. To truly know how to build your deck, you need to decide ahead of time what you're trying to do with it. Most decks have their image set up around a grade 3 unit, because speaking realistically, the process of dealing damage takes enough time that you aren't going to win until you hit grade 3.
In addition to the time factor, grade 3s have their greatest strength built into them. Twin Drive as a skill provides raw card advantage, as well as the possibility of multiple triggers activating. With very limited exception, most grade 3s have incredible skills unique to them that can completely turn around the tide of a match.
As many grade 3s make the most of their skills when supported by rearguards designed for them--Soul Saver Dragon and Pongal, Tyrant Deathrex and Dragon Egg--these units should be, along with one's starting vanguard, among the first cards placed in a deck. The winning image brings your strategy together, as most cards do not stand on their own but instead stand together. While there are certainly exceptions like Oracle Guardian Apollon, who can be brought into any deck and doesn't rely on specific clan skills, this is the ultimate core of Vanguard's strategy.
Cardfight is not a game of which cards are the strongest; it's a game of making the best of a particular strategy. While it is possible to combine Blazing Flare Dragon and Dragonic Overlord The End, a deck which focuses on one of them in particular and plays those cards to the best of its ability will always do better than a deck which clumsily attempts to unite these.