Zinnias, sunflowers, 4 o'clocks, to name just a few.

Hollyhocks, four-o-clocks, digitalis or foxglove, balloon flower are just a few more I usually start and find easy.

Are you using a light setup or sowing outside? Dahlias are real easy but need an earlier start.
Touch-me-nots, forget-me-nots, shasta daisy, salvias, amaranthus, calandula, marigolds, columbine to name a few.
Anyway, what does well one year, might not the next. You never know with seed growing. Expect some failure, possibly a lot. As you go along, you learn to troubleshoot a bit as you figure out why your plants perish.
You might have a ton of sunflowers one year and have all the rodents eat them the next. Or something germinate heavily one year and nothing the next. Or have them all rot in a cold wet June. Or have something germinate wonderfully, grow your little plants, put them in the ground and find out it is a bad year for cutworm or slugs or beetles or something you will never figure out for sure. You may ask yourself why do I bother? By then though, you already bought the seeds so you just go ahead and do it again.
It also helps to search google. Try using germination with your plant name. Some plants are easy but have certain requirements like freshness, warm and cold stratification, scarification, etc.
Anyway the results can be rewarding.

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I call this one Angelina because the day I noticed its first bloom was the day my neice was born. Perhaps a real dahlia grower would have plowed this one under because the orchid form is not perfect and it could be 7' tall. But since I grew it, I get to keep it and I haven't found one like it for sale locally.

Nicotiana, hollyhocks, digitalis, cosmos and zinnias are the easiest ones I've ever done.

It sounds like using winter sowing can improve your odds with some seeds. I plan on trying it this year for the first time, but I've already benefited from it by getting some great plants grown this way from plant swaps.

Here is a link that might be useful: Winter Sowing forum

Morning glories -- but they can be quite prolific so take heed. Mesculine, peas, string beans were successfully grown from seed in my veg garden this year.

Hi
For summer sowing, I've always had luck with nasturtiums--they like rather poor soil on the dry side and lots of sun. If the soil is too rich you will get more foliage than flowers, but other than that they are easy and colorful.
Pat

I direct sowed hyacinth bean this year. Also started ornamental peppers indoors with the tomatoes. They really did nicely. Hibiscus moscheutos are easy too - the direct seed method in the fall.

verbena bonariensis. It reseeds itself easily.