I use three main timbers. Each is right for different circumstances.
Engineered Redwood is the base timber for our gates are made from. It's stable, takes paint and primer extremely well, machines cleanly and - when properly finished and maintained - will last twenty-plus years without drama. The engineered dimension means it's far less prone to the warping and splitting you get with poorly selected solid softwood. This is our standard timber for a reason.
Accoya is the premium option. Modified Scots pine with a 50-year above-ground guarantee against rot and insect attack - that's not a marketing claim, it's a certified product specification backed by a real warranty. It's dimensionally stable to a degree that genuinely impresses me even after years of working with it. The cost is higher - roughly 60–70% more than Redwood for materials - but for clients who want to buy once and not think about it again, Accoya makes sense. Our full Accoya vs Redwood comparison is here.
Iroko is a dense African hardwood. Naturally oily, naturally resistant, excellent for clients who prefer an oil finish rather than paint. It has a character that grows on you - the grain is slightly more irregular than the engineered timbers, but for period properties particularly, that works in its favour.
What I'd steer you away from: untreated softwood from a builders' merchant, anything described as "treated pine" without specification of the treatment standard, and cheap composite gates marketed as timber alternatives. I see the results of those choices regularly. They're never pretty after three winters.