There are lots and lots of good .40 and .46 size engines out there, and everyone has their favorate. There really aren't any absolutly horrible engines in that class out there right now, as far as I know.

For beginners, I happen to like the Evolution line of engines. Very good power, inexpensive, and easy to get running right.

OS is widely thought of as one of the best engine makes out there. You'll hear mixed reviews of the LA line. They don't produce as much power as most engines in their class, but they are easy to run and reliable. The OS FX/AX lines are powerful, reliable, but expensive.

Thunder Tiger Pro series is a good engine in that size. I don't see them much at my field, but the guys I know who have them love them.

There are a lot more out there. Assuming you are going to learn with an instructor, it would pay to check out your local field and ask what most guys there are running. Most engines have some quirks, so using an engine no one at the field has seen before could be an issue.