Part assortments for creative building
It very much depends on how and what you like to build so personal recommendations are difficult. As you're new-ish to MOCing it may take some time to figure out what pieces you like the most.

That said, I'll offer a suggestion - I find 10230 Mini Modulars and interesting range of small pieces in various colours incuding some rare ones. Check out the inventory to see if it appeals to you too. I'd also suggest looking at the Creator sets in general.