As far as caliber, the .45 ACP is still the largest of the commercial loads. It makes a bigger wound channel than the .40, or the 9 mm.
I like a quality 1911. A 1911 in .45 was the infantry side arm for our us forces for 60 years, and still serves elite forces.
I will say that a striker fired gun such as a Glock, or a Springfield Armory XD is an easy gun to master with little training, whereas a 1911 needs some work to keep it safe, or operate its single action trigger.
I whole heatedly will recommend a .45 caliber, but the model is up to your preference. Get a big 5 inch barrel for the max in velocity and sight radius.
Some makes to consider are Springfield Armory 1911s as well as XD, Glock, Sig Sauer, Ruger, and my favorite a Kimber SIS 1911.