Hunting for the right crossword fill, I usually think in tiers — the short, high-frequency ones first, then the fancier long options. For a four-letter slot, my go-tos are 'APEX', 'ACME' and 'PEAK' because constructors love those; they fit a lot of intersecting words and are nearly default answers for 'pinnacle'. For five or six letters I'll try 'CREST', 'CROWN', 'ZENITH', 'SUMMIT' or 'CLIMAX' depending on crossing letters. 'APOGEE' and 'VERTEX' are nice if the puzzle leans a bit scientific or more formal.
I also watch the clue tone. If it's literal — mountain or orbit — 'SUMMIT' or 'APOGEE' fits. If it's figurative — career high or artistic best — 'ACME', 'APOTHEOSIS' or 'CULMINATION' (if the grid is long enough) feels right. Cryptic puzzles sometimes use wordplay like 'top earned by a king? = CROWN' or synonyms reversed, so think about short common fills first and then graduate to the rarer, longer words. Personally, I keep a mental cheat-sheet of the top 10 fills and it saves me a lot of time — feels almost like hunting treasures in a familiar map.