I cycle less than 10kms a day but I'm a little on the heavier side as well. What would you recommend, Aluminium, steel or Carbon fiber frame (body)?
The material doesn't matter so much in this case -- the design of the frame is more important. You aren't looking to save weight on the frame or asking a lot of performance of it (e.g. you're not sprinting in the Alps).
For leisure riding, you want a relaxed riding position which is usually more upright than a racer. You can find hybrids (e.g. Specialized Sirrus/Trek 7.x/similar) made from steel/carbon fiber/aluminum (depending on how much you want to pay). Good bikes can be made from all of the materials, but, of course, a bicycle is the sum of its parts, and the more expensive ones typically come with other improvements as well (better quality wheels, shifters, nicer fork, etc.).
At least on the US market, for most people looking in the hybrid-type bicycle class, most of the bikes I'd suggest (Specialized Sirrus/Trek 7.x) tend to have aluminum frames and carbon fiber forks at just over entry level price points (with the entry level versions having aluminum forks).
Given what you've said about your riding, an Aluminum frame should suffice - although if your goal is to cycle more, buying the best you can afford might help you enjoy it better, and therefore get out on the bike more frequently.
When it comes to frame materials - the late, great Sheldon Brown said it best, however:
Did you know that:
- Aluminum frames have a harsh ride?
- Titanium frames are soft and whippy?
- Steel frames go soft with age, but they have a nicer ride quality?
- England's Queen Elizabeth is a kingpin of the international drug trade?
All of the above statements are equally false. There is an amazing amount of folkloric "conventional wisdom" about bicycle frames and materials that is widely disseminated, but has no basis in fact.
The reality is that you can make a good bike frame out of any of these metals, with any desired riding qualities, by selecting appropriate tubing diameters, wall thicknesses and frame geometry.