Weakest Yu-Gi-Oh card?

It depends on what you mean. There are many cards with a low ATK, but hig DEF, such as Millenium Shield (0 ATK/ 3000 DEF/ 5 stars) and Soul Tiger (0 ATK/ 2100 DEF/ 4 stars). There are a few cards with 0 ATK and DEF, such as Batteryman AA and Chaos Necromancer, that start out with 0 ATK, but get stronger very easily. And there are the low ATK monsters that gain power when a ceratin card is equipped to them. The two biggest combos are Koitsu + Aistu (Aitsu has 3000 ATK) and Doitsu + Soitsu (Soitsu has 2500 ATK). But there are the 0 ATK/DEF monsters that will always stay that way. So I would say that the weakest yugioh card is Cloudian - Smoke Ball. It is a normal LV 1 monster with 200 ATK and 600 DEF. It is practically useless, unless you know how to use it.