We went to a display place and tried all three out today. The winner was the Navigator because you can attach a tall climbing tower (slightly rocket-like in shape) to it. However, the tower is now discontinued (we have bought the display model) and it is rumoured that the Navigator is being phased out. The Navigator is the only one you can attach a swing to (used to be able to buy swing attachments for the Challenger but they aren't available anymore) which is another thing that was in it's favour (but dh has just 'won' a big multi-swing thing from ebay so we don't need a swing arm anymore).
The kids weren't so happy with the Challenger main frame since the den is in the middle so there are no 'monkey bars' to dangle from as such...you can buy monkey bars that attach at right angles to it though.
My personal fav was the other one..ummm...explorer...with additional monkey bars (you can get a 'tiger run' which is for walking over rather than dangling from but it looked a bit tame for my kids). If I had known that dh was going to get the mega-swing I would have simply ordered an explorer without consulting the kids As it is, they fell in love with the tower for the navigator.
I am hoping I can draw to a close the TP obsessed episode of my life now
Although...now they want a TP trampoline