I've had the best luck with Giants of California. I now save seed each year, but originally purchased my seeds from (gasp) Ocean State Job Lot and Walmart.

I've also tried Zahara, Pinwheel, and Giant Cactus - they did fine. but the Giants of California really thrive for me.

Hi NEgardener, I've grown the shorter Profusion and Persian carpet type Zinnias, but found the butterflies don't like them as much as the tall Zinnia elegans cultivars. I've grown the California Giants, also Cut & Come Again, State Fair, Scarlet Flame and now I collect my own seed too. Prefer the single and semi-double varieties because the nectar tubes are more accessible to the butterflies.

Zinnias are heat lovers and won't grow much until it warms up in late Spring. I direct sow mine around June 1st. They grow fast and by mid-July start to bloom.

Here's a pic from September of last year. There is some Tithonia 'Aztec Gold' in the background.

Hi,
Last year I started to plant this variety. It bloomed the earliest in my Garden in mid-July, 3 weeks before other Zinnia and Marigold. And it kept on blooming till frost.
All zinnias are hardy and colorful, as well as marigold and cosmos. So this year I plan to try pinwheel and northern lights(?), just for fun.