yeah, definitely crater, i recommend left path. if you're trying to master materia specifically, i recommend using cloud with apocalypse and cid with scimitar, both have triple materia growth.
also, there is an armor with double materia growth that you can buy somewhere along the lines, i don't remember where though.
i usually have cloud and cid with triple materia growth weapons and barret with the double growth armor, but that's my preference.
run from everything other than magic pots and movers, the rest aren't worth fighting, unless you need to steal elixirs from tonberries