Most versatile word in English?

An argument could be made for OK but Set has the most separate entries in the Oxford English Dictionary (463). It is very versatile and takes a number of different grammatical forms including:
  • concrete verb e.g. set an alarm on a clock 
  • abstract verb  Mr Smith is set to take over as Prime Minister 
  • concrete noun a box set
  • abstract noun a set of problems. 

Phrasal Verbs

There are also large number of phrasal verbs based on set. A phrasal verb is an idiomatic expression in which the meaning of the original is modified by an adverb (set down) or preposition (set to/set off). 

Phrasal verbs are picked up quickly by L1s or native speakers and are often considered easier to understand: a child will understand set out before they learn commence. 

The flip side of this apparent informality is that phrasal verbs are particularly challenging for English language learners. Some offer little clue to usage -  to set about changing for example. 

The versatility of set demonstrates the flexibility of English and its capacity for nuance and ambiguity