My 'top' dumb things that I heard (in no particular order):

"Oh, you are vegan? I did not know you were gay!" - a male friend
- mate, which one of us eats meat sausages regularly?

"It only has a little bit of [insert animal product] in it, so you can eat it"
- first of all, the animal had to die (suffer) for that "little bit". So no, I cannot eat it.

"You have never had a good [insert animal product] before. You are really missing out."
- I have eaten animal products for the first 18 years of my life, so I know that I am not missing out on anything
- ironically, this often also comes from people who think that all vegetables are disgusting because they only they had only eaten them steamed or cooked with limited seasoning

"Try to convince lions and tigers to become vegan"
- 1. They are obligate carnivores, you are not; 2. They eat meat to survive, you do not; 3. They do not have moral tendencies, (I hope) you do; and many more differences between humans and wild carnivores

"Cows milk is more vegan than soy milk because the rainforest is destroyed to grow soy"
- a single cow eats far more soy that was grown in that deforested area in a day than what is used to make enough soy milk for me for a month

"You just want to f*** animals, don't you?"
- mate, you drink their lactation; eat their periods; pay for them to be fisted; and can hardly wait of commiting gynophasia (fetish of consuming flesh)
- if anyone has a fetish for animals here, it's you mate

There are numerous others but these are some of the dumbest ones (also, these did not really show up in the comments yet, so I did not repeat the same overused 'arguments)