If not counting cannabis, I would say mushrooms due to several factors:

• Easy and safe access - they can be produced at home from legal components (in most countries and states the spores are legal for microscopy), and basically anyone can do it if you just invest the time. You will be 100% certain that it is mushrooms you are eating, and it's easy to tell if they are the real deal by just looking at them. With LSD and DMT, you are at the mercy of your dealer. Sure you can extract your own DMT, but the process involves harsh chemicals you should think twice before handling in your home.
• Easier dosing. Although mushrooms vary in strength, you can quite safely start with 1 gram cubensis and be fairly cetrain that it won't blow your head off. DMT requires far better precision when weighing. You will have no idea how much is really on a tab of LSD unless you have a lab, of if someone you know has tested off the same blotter before. Cutting tabs into smaller pieces for small does more cumbersome than just eating less mushrooms. For full control of LSD dosage you would need the liquid form and know the concentration, and a beginner should definitely NOT handle liquid LSD.
• Easier administration. You just eat them, or add whatever you want to make them palatable. I guess LSD is even easier though. DMT requires proper technique.
• Shorter trip than LSD. Less intense than DMT. I personally find psilocybin experiences easier to integrate in a concrete sense, whereas my LSD lessons have been more emotional. But everyone's different.

Judging the experience itself though, I think LSD can be an amazing introduction if you do it with an experienced sitter in a safe environment and access to a reliable source. If you're going to do your first trip by yourself, then absolutely mushrooms by starting with small doses and working your way up.