They might just be the most cheerful flowers on earth!
Anyway, I love 'em and always have a few, plus a few extra because sometimes the seedlings are devoured by my spring snail crop and I don't ever want to miss the flower power that really takes off in the late-summer/fall. (of course I could say the same for alyssum, and their volunteers, plus they smell good and aren't too thirsty)

I really do love zinnias also. They are my favorite annual.

Somehow Alyssum totally escapes being eaten by my Slug Army.

That was how I found them: I planted a mix with dozens of flowers in it. The only two that were not eaten were Alyssum and Siberian Wallflower.

Siberian Wallflower (Erysimum marshallii)

Zinnias seem survive with only a few bites, but I've started them indoors, and only put them out when pretty big. That isn't necessary with Zinnias, but it does frustrate the slugs.

Have lots of slugs here also but have never noticed it stopping the zinnias from growing.

A few years ago I planted a flat of good-sized zinnias around the porch, plus here, there, and everywhere there seemed to be a space. Next day they were gone. Just nubs. Then I discovered a bumper crop of snails--big ones, lots of 'em--and they must have overwintered and woke up hungry. They weren't eating the daffodils or the yuccas --go figure--but they devoured the zinnias immediately.
Constant battle with my irrigation water. Too bad there's not a market for the snails in these parts. Oh well, the birds enjoy them--smashed and fried on rocks.

Where I live, slugs seem to made of anti-delphinium. Forget starting them in the ground, or even hardening them off on a table on the porch. As soon as i put a seedling outside and turned my back, they disappeared to the soil line.

I had to put them out much bigger and surrounded by slug bait. Once the plants got big enough (?? 1-2 feet??) they became safe and didn't even collect "slug bullet-holes".