If there are no prey conditions, does an enemy get assigned to whomever the lead investigator wishes?
Close. If you draw an enemy (e.g., from the mythos encounter deck or your own deck), then prey instructions are always ignored -- the monster automatically engages whichever investigator drew the card.  The lead investigator has no say in the matter.

However, if there are two investigators at a location, and an enemy moves into that location (e.g., from the hunter keyword) and has no prey instructions, then the lead investigator can choose who the monster engages with.

In the RRG, this is covered under the event details for the Mythos phase:


  If the card is an enemy, spawn it following any spawn instruction the
  card bears. (A spawn instruction is any text bearing a "spawn"
  precursor.) If the encountered enemy has no spawn instruction, the
  enemy spawns engaged with the investigator encountering the card and
  is placed in that investigator's threat area.