Organic mulch is usually the best choice because it blocks light from weed seeds, helps soil hold moisture, and adds organic matter as it breaks down.
For most garden beds, we use about two to three inches of mulch. That is usually enough to reduce weeds and keep the soil moist.
Around trees and shrubs, we always keep mulch a few inches away from the trunk or stems because piling it against the base can hold too much moisture and lead to rot.
Shredded bark or wood chips work well for landscape beds because they last longer.
Compost, shredded leaves, or straw can work well in vegetable beds because they break down faster and improve the soil.
More mulch is not always better. A thick layer can block water from reaching the soil. Instead of adding a heavy layer every season, we check the existing mulch first and only refresh what is needed.
Good mulch protects the soil, but don't bury the plants.