Mine are:
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Wiktionary. Every month,
this GitHub packages the entire English Wiktionary into a StarDict format for use with KOReader. It's formatted great and works great.
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Oxford English Dictionary (OED), the behemoth. A great dictionary, but maybe not so suited for KOReader due to the length of each definition and the time it takes to scroll through them to find the right sense of the word you're looking for. But certainly the authoritative voice, the most words available, and my absolute favorite. A fellow MR person optimized it for KOReader; it can be found [
Snip link to unauthorized copy](I once owned the Concise OED in 2 volumes, and they don't provide an offline version, so I feel good about having a copy offline for my e-reader in this format. Ask yourself if you feel the same.) I've made further optimizations for it in KOReader but haven't had the chance to upload my version anywhere.
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Shorter Oxford English Dictionary (SOED). This is the IDEAL dictionary for KOReader in my opinion. It has the exact same definitions as the OED above, but the language has been made more concise without losing precision. And it doesn't have all the attestations. It's much better for casual lookups, without losing the OED's definitions. I own this IRL and on various digital formats (Mac app, iOS app). The only cons: it doesn't have quite the same coverage as the OED and leaves out some nonce or near-nonce usages (i.e. senses that are only used once).
So all in all, the SOED is my favorite because it follows the OED practice of listing senses in the order of their historical development. The oldest meaning first, the newest one last. Helps identify which meaning the book you're reading meant depending on what year it was published, and just plain better from a linguistic perspective.
Wiktionary is good for up-to-date etymologies, but the ordering of the senses is essentially random so I don't like it as much after finding the SOED.
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Only problem is that the SOED doesn't exist in a well-formatted StarDict format anywhere. The existing one floating around uses images instead of bullets, and no one's taken the time to fix it. I tried and had some success, but was faced with the further issue that the entries are not indented properly and just jumbled up. Newer digital versions (like the Mac version) have the entries in their proper indentation and HTML nesting, but it's hard to extract the HTML to create a StarDict since the underlying database's definition blobs seem to be encrypted even though it looks real nice in the app interface itself. Please reach out if you have a nicer StarDict or HTML version of the SOED on hand, would be cool to optimize it for KOReader.
Merriam Webster and the like are complete crap because they quite literally change definitions on a whim based on recentist and localist political winds and donor money (probably because dictionaries aren't making a lot of money otherwise these days, since everyone expects them for free, similar to newspapers), which I find quite dystopian and extremely unreliable. Some old dictionaries like Johnson's are cool too because they're written in such a casual manner that makes them fun to read, but I haven't found good StarDict versions of them yet.