Best frame material for Leisure
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).