What circuit should I use for a new outlet in my living room?
IMO it makes absolutely NO sense to use an existing circuit, especially considering how taxed they are. As long as your panel will accept tandem breakers what is the issue? Also, with a house of this vintage I highly doubt the wiring is grounded, which you cannot extend. And do not assume if the wiring is old AC/MC cable that it is grounded, much of it from that vintage is not, regardless of the metallic sheathing. The only metallic wiring like this that can be used as a grounding conductor is that with the think aluminum bonding strip running along side the insulated conductors.
Besides, you never know what you might use this new receptacle(s) for. Don't think you'll never use it for anything more than a laptop charger.

Also, you re-purposed the old A/C circuit for a dryer?? Was it at least 10/3 cable? If not you have a non-complaint and potentially dangerous circuit if used for a 120/240V electric dryer.