I'm looking to build an outdoor swing set for my kids in the garden. I'd like to know what is the most durable wood to use that doesn't warp or bend. I've been down to the big department stores to get wood, but it is mostly warped already and it has been difficult to find a quality wood.
I would highly recommend Cedar. Similar to Redwood, it is resistant to insects and rot. If you can source it, Eastern White Cedar is a wonderful wood to make a play set out of, as it wont give splinters as easily as many woods will.
I went a bit overboard on the play set I built last summer. I went with rough cut western red cedar 4x4s, and jointed/planed them down to about 3 3/4" x 3 3/4". This allowed me to end up with very square members. This was necessary as I did everything using mortise and tenon joinery (yes, I went overboard).
The primary problem with cedar warping/twisting is when it is unsupported. Once supported properly (i.e.- assembled as a structure), the warping/twisting is minimal.
If your budget allows it redwood specifically heartwood. It's resistant to insects and rot and gives a good finish! If you need something a bit cheaper go for cypress again resistant to insects and rot. The costs go down but you do loose out a bit! After that its really a case of pressure treated woods which I don't have a lot of faith in!
I built this swing about 8 years ago and if I remember correctly I just used Pine, because it fit my budget at the time. If I were to do it again I might have spent more and used redwood or cedar as some others have suggested. When I used to build decks we would always try and use redwood or trex, but both of those are more expensive.
One thing I did do that you should do no matter what kind of wood you use is to seal it. I used a really good protective paint that also contained sealant and some UV protective solution and the swing is still in pretty good shape after 8 years. However, this swing is pretty beefy (I used 6x6 posts and 6x10 beams) so warping wasn't really that much of an issue for me, just a little splintering here and there.
Another thing you can do is put some Boracare on it. It's perfect for keeping all wood-eating pests away from wooden outdoor structures, like swings.