What is the best martial art for a big man 6ft4 270 pounds who isn't that athleticaly talented?
Bear in mind that any legitimate system of Martial Art, and any good school is going to be beneficial to a person regardless of age, gender, size, or even physical abilities. Some methods of physical combat focus on aspects that might prove to be more challenging at first, or might need a greater degree of adjustments to fit a person who is tall, large, or physically challenged. However, being "athletically talented" should not be an issue since it is the path of any student to start at the beginning, where nothing is expected of you except a willingness to try your best. In time, you will develop the talent and skills that you currently lack, or have not yet discovered within yourself.
Many of the commonly known Asian systems of Martial Art were designed to apply natural forces to allow a smaller, and weaker person to defend against a larger, and stronger attacker. One of the greatest challenges for a larger student is to avoid using brute force, and learn the advantages of finesse, balance, leverage, and increased power through scientific principles.
Larger people might find the nature of judo and aikido to conflict more with their size simply because much of the leverage in throwing, projections and manipulations require the defender to place their center of gravity below that of their attacker. Other than that, the rest of those systems can be learned with relative ease. There are many large people who do well in both judo and aikido, but some adjustments must be made. Many lager people do well in methods that focus on striking, such as Karate, or Taekwondo.
The best thing to do is to visit a variety of well established schools with good reputations, watch other students in action, ask questions of the instructor, and request to try a sample class to see how it feels to you. Most new exercises and activities are going to feel "awkward" and be challenging at first, but it should be something that you can adjust to over time.