[alkyon]

Probably it's Middle Liddel, I haven't decided to buy the unabriged version due to its unwieldy size, high prize and because it is 80 years old. Apart from this, it's fully available online.

Just started relearning Ancient Greek after twenty years and I highly recommend Cambridge Greek Lexicon.

[dhosek]

Generally, I go with wiktionary which is reasonably comprehensive. I remember as an undergrad being stumped by a word in the Septuagint that I could not make any sense of and now I imagine I can search on the inflected form on wiktionary and know exactly where I went wrong. (I would note that as an undergrad I was also pretty thoroughly defeated by Attic Greek. I’ve since learned that the textbook we used, Hansen & Quinn, is pretty rough going, but I’ve also found that the approach taken by the Biblical Greek textbook I worked through (N. Clayton Croy) made what was nearly impossible forty years ago a breeze on the retry nine years ago.