There is a reasonable assortment of banks in Guatemala as well as Credit Unions (Financieras) — each with their own advantages. Here are the ones I have dealt with:

• Banco Rural — Their claim to fame is that they have more locations than any other bank. If you are just working in local currency (Quetzales), great but they offer a much worse exchange rate than other banks between US dollars and Quetzales. USD and Quetzal accounts.
• Banco G&T — A reasonable number of branches and an OK exchange rate. USD and Quetral accounts.
• Banco Industrial — Likely the biggest in assets, OK exchange rates and a decent assortment of on-line services. USD, Quetzal and maybe Euro accounts.
• Micope — This is a collective of financieras. You can pick a local financiera (for example, Guadalupana in Guatemala City, UPA southwest of the city and COLUA in Sololá) and get the backing of a bigger organization and ability to access your account from other MICOPE members. Many members only have Quetzal accounts.