There are many combinations of fly rod and reel that are great and functional. To choose which combo is best ask yourself these questions.
What type of trout will I be fishing for.
What size trout will I be trying to catch.
Is there a specific method I will be using more often, dries, streamers, nymphs.
What is your budget?
The best selling of all is a 9 foot, 5 weight. This size rod is an all round size good for most size trout. When targeting smaller trout it is common to use a shorter and lighter fly rod in the 2-4 weight size. There are specialized nymph fishing rods also that are in the 10-11 foot size.
Many experienced fly fishermen will carry two or more rods when out for a day on the water. There selection typically includes a dry fly rod along with a rod suited for throwing heavier streamers.