I will be staying in Guadalajara Centro around Calle Joaquin Angulo and Calle Contereras Medellin in January, and I'm wondering if anyone can suggest the best ATM(s) to use in that area? I assume it's best to use an ATM attached to or inside of a bank? Any advice with the most trustworthy locations with the lowest possible fees is greatly appreciated.

I was recently in the Aranzazú area and used a Banbajio atm. They're pretty much my go to atm since they dispense a full mix of bills (500, 200, 100 & 50) when I withdraw up to $8000 mxn at a time which about matches my US bank's $500 USD daily limit at the current exchange rate. $23.20 mxn fee. Santander typically has a low fee, usually in the $35-$38 peso range and are usually more ubiquitous. Best to avoid Bancomer & Scotia, they have higher fees. Banorte is good too. It's not necessary to use a machine attached to or in a bank. Most supermarket chains such as Bodega Aurerra, Walmart express, Soriana, Comer etc will have a machine or 2 if there are any near your area, and those locations would be fine & secure. Whatever machine you find/use, be sure and decline the initial conversion prompt/screen that offers to show you your withdrawal amount in your home currency. After declining, the dispense will continue.

Generally any ATM within a bank or large commercial store (Walmart/Chedruai) will have the best rates. What you should completely avoid are any ATMs that disperse USD currency. Not only will your withdrawal be subject to exchange rates and transaction fees, but you'll also need to exchange the cash afterward for an additional high fee.

Thank you both for the helpful information. It's greatly appreciated.