Actually I disagree with the concept of this topic cause in YuGiOh ALL cards are "must have"... What cards are you going to use more is entirely up to your own taste of battle.I am using warriors so it is imperative for me to have at least 3 copies of greenkappa and another 3 of eyermor cards,amongst others.If you like machines for example then having a greenkappa isn't exactly a priority for you,it can show up as the last card missing from your library and it would still not affect you game...
The "must have" expression can be applied on only one thing about this game.Should you choose a particular monster as a deck leader then having 4 copies of it IS a must have.My library is complete and I only have one copy of the Droll Bird so I can not pick this monster as a low level winged beast deck leader and upgrade it to SD. --- one omen is great.fighting for your country.
"Must have" depends on the deck you're using. For example, my first deck revolved around Mystical Elf and Hoshingen powering up my other light attribute monsters, so I needed two or three darkness Approaches. Often I could power up a monster by over 5,000 points in a single turn..
Depending on your strategy there are certain cards you need. If you run an Exodia deck then Overlordtico's recommendation would be ideal.
In general if your strategy is to reduce an opponent's LP trhu battle then the cards you "must have" would be Tears of the Mermaid, Mesmeric Control and Shadow Spell. These cards will not only reduce your opponent's monsters in power but also hold them in attack position long enough for you to destroy them and do damage. While I don't like using cards that need a password to obtain, Mirror Wall more or less guarantees you victory if you use it and can protect it.
This game affords a lot of variety in terms of winning deck types. Light, water, Exodia and zombie decks seem to work really well but some decks based on individual monsters like a fusion/ Muka Muka deck can give you a monster with an atk of 9,999 in a single summoning.
Just play around and let us know what you come up with.
Good Luck! --- Born in darkness, living in darkness, knowing only darkness, the shadows are my only allies.
lol, most my decks got 1 goal or 1 theme. I barely ever make anything of multiple strategies in one. It makes the draws more inconsistent --- PSN - overlordtico http://youtube.com/user/underlordtico http://dbcompetitive.forumotion.com/forum.htm