What's the best plant along a 3.5 foot fence, lots of sun?

Lantana

Roses would love that area, if you mulch them well. Any sun loving plants would do well. I was taught to plant tall flowers in back, and shorter in front, so you could plant various types and sizes. Petunias, Begonias, geraniums, raspberries, strawberries, cosmos, and many more would do well. You might check with the local nurseries and see what plants show a sunshine on their tags. Most plants are marked with tags to let buyers know sun or shade. You can find the sizes, textures, and types that appeal to you personally.

Marijuana loves the sunlight

Since you have a low fence, you may just want a low growing plant. My go to plant is vinca flowering ones, not the vine. They seem to grow nice where ever I put them. The down side is that they are annuals.

If you have sun you have a choice of so many kinds of shrubs and bushes. Get a flowering kind because they are all beautiful and would florish there. Good luck