- For Teachers
I'd opt for the first. ;-)
The present perfect tense (as in "I've enjoyed") is always used to talk about an event that happened (or or never happened) at an indefinite time in the past - e.g., "I've been to Mexico". As soon as the event is fixed at a specific point in time, as in "I was in Mexico last month", the simple past tense is used (as in "I enjoyed").
In the present case, the context implies that we're clearly talking about something that just happened - this last time that I saw you. But if I were to say "I have enjoyed seeing you", it's (gramatically) unclear as to whether I am speaking about the most recent visit or some other visit in the past. For all my interlocuter knows, I may be implying that "I have enjoyed seeing you before during other visits, but on this last visit you annoyed the bejeezes out of me!"
So to avoid any misunderstandings and getting bopped on the nose by an insulted guest, I'd chose #2.