I stock a few popular preservatives. Back in the day when I started creating products, benzoin preservative was the only one readily available, and it is still my preferred choice today even though there are now more options out there.
I love the smell of Benzoin tincture, and as I am generally working with essential oils and creams/ointments, it helps act as a fixative, keeping the oils in the product rather than having them evaporate every time you open the cap!
Most presevatives will need to be combined as they typically only work with oil or water, and their focus is on mould or fungi, however phenoxyethanol will do both so comes highly recommended in a lotion that is more water in oil.
pH plays a factor too... it's a steep learning curve if you never studied chemistry in school, however once you have mastered it you will be very excited about what you can create and how long it lasts for.
There is more information under each product.