Okay so the table of special abilities on RM Companion 1?  Some grant a single spell list to 50.

I am sure the answer varies based on your profession...but which one would you pick and why?  And oh let’s keep it to Core and RMC1.

I was actually considering the Closed Channel Creation just so my guy could play the party chef.

Not an answer to your question but it made me think about a ruling I made when a player ended up rolling this as a random BG Option.

In order to curb casting multiple high level spells each combat round with this in tow, I ruled that whatever difference in Caster Level vs Spell Level was, was the amount of days that would have to pass before one could use the spell list again.
It made sure that I wouldn’t have to scrap all BBEG encounters or juice them up to become potential insta-TPKs.

Not to derail this thread, but It is interesting you say that:  I vividly remember there being a note saying something to the effect of “not needing to make ESF rolls for” but in looking back at the original RoCoI, it does not say this at all.
I wonder what influenced me to think it needed to be curbed - perhaps it was seeing the same ability in one of the later books that just stuck in my head.

Ethereal Mastery - 3.14 RoCoI

I mean, it has a lot going for it: it's Arcane magic (i.e. powerful magic - look at the note at the top of the spell list: "It should be noted that this list contains among the most dangerous spells ever researched"); it has teleportation spells; it has spells to move from the material plane to the ethereal plane (and by extension: onward to other planes of existance); it has the Ethereal Room spell (how awesome is this spell for hiding stuff from other people?); it has see invisible spells; it can conjure entities from other planes; and most powerful of all, it can destroy magic items (with the 50th level spell: Disjunction).  It's one powerful list.

Nightblade ->--

p.s. as for spell restrictions with the BG Option from RoCoI, that's the only way to keep play balance, otherwise you have PC's casting high level spells when they are still on low levels.