Plain Gravel
This is not a recommended substrate unless mixed with some sort of nutrient rich additive or soil, at least a 50/50 mix to be effective.
Fluorite/Seachem
These are great nutrient rich substrates which are not cheap but will provide good minerals and vitamins for your plants.
ADA Aquasoil (Amazonia)
This is great not only for plants but will be perfect for small foreground plants to grow in. In addition it will drop the ph to just under 7, this can be good or bad so check before you use this substrate.
Clay or laterite
This substrate can also be very effective if topped with a gravel but the problem is, it can be messy and leave the aquarium very cloudy, whenever plants need to be moved it may be an issue, unless you know what your doing you should probably leave this one.
DIY soil/fertilizer mix
This is a great choice and means you can make the substrate at a very reasonable cost, there are countless guides on how to do this, but for the record do not use any soils that are full of fertilizers or phosphates as these could be detrimental to your aquarium.
Other commercial Substrates
Eco Complete/Up Aqua or any other types of aquarium substrates should do the job great, just be sure to read if they will change your water qualities or ph so you don’t end up with any problems.
Remember it’s not the end of the world if you put in a substrate which doesn’t work the way you wanted, commercial substrates are expensive and so every day people come up with new and improved recipes for their substrates, don’t be scared to try this either.