It is totally up to the type of use or the type of interpretation that you want to give to the theoretical price and also it depends on what are your belief about the information content of the queue of orders that are currently resting and are visible in the limit order book.
Depending on whether or not you believe that the information displayed in the limit order book adds to your final result for the theoretical price, you could take:
- the last price at which the security was traded;
- the mid-price calculated as an unweighted average of the best bid and best ask prices;
- the mid-price calculated as a weighted average of the best bid and the best ask prices (depending on what is your belief about information content of the depth of the LOB on each side you could decide to include only level 1 information or use the entire depth as in Cao, Hansch, and Wang (2004) 'The Informational Content of an Open Limit Order Book' ).
However, in one of your comments I read that your interested in how a market maker would price that security, and in that case the answer may not be a single number since they are requested to post quotes on both sides of the LOB and therefore they will have a theoretical price at which they believe it is reasonable to sell the security and a theoretical price at which they believe it is reasonable to buy the security and based on that they will post their quotes in the LOB (take also in consideration that they are not obliged to post at the top of the LOB so their theoretical prices may differ from the best bid and ask).