It's avocados. You don't spell tacos as tacoes.
... but you do spell the plural of potato as potatoes
If I am not mistaken, Avocado is a Spanish word, therefore its interaction with English grammar is not the same as a word like "potato."
Potato and tomato are also from Spanish so that argument is invalid. I've been trying to work out why this is so. All three come from the Andes region so why do we put an 'e' with the two and not with avocado? Weird. I don't really understand the difference between heroes and zeros either. /shrug. The English language is weird, though!
I would suggest it is the timing of entry into the english lexicon. Potatoes and tomatoes have both been around in english much longer than either tacos or avocados.