I would facet against a field to discover the most common text, phrases or words (depending on your index settings) are that appear within Elasticsearch' Inverted Index.
Sample Facet
{
"size": 0,
"query": {
"match_all": {}
},
"facets": {
"tag": {
"terms": {
"field": "firstname",
"size": 6
}
}
}
}