It depends on what you want to do.
If you just want to scout, you can use all, for free.
If you want to scout and check how players develop, you need to use Genie Scout and if you play very fast, you need the "g" version, aka pay for it
FMSE, FMRTE and the in game editor are need to be bought to be used to change a lot of things in the game, from tactics knowledge to players attributes, etc.
I always use Genie to be equipped with the same tool that the AI has: knowledge of the regens that appear before the AI snatches them. Usually, the AI knows where the best regens are, don't know how, but its a big coincidence.
The other editors i use when im in the 10th season and want to have some fun, so use the one who has the best things for you. Last year i was on FMRTE, this year im going to the in game editor. just because.