The official Indonesian-Indonesian dictionary from the national language agency.
Details
KBBI is the authoritative dictionary of Indonesian, maintained by the government language agency, and the online sixth edition is the reference to settle what a word really means, which register it belongs to, and whether it is standard at all. Because it is monolingual it only becomes usable around B1, but from there it beats every bilingual option. You need a free account to search the web version; offline mobile apps exist if you would rather not register.