All have their pros & cons.
Aluminium is lightweight, strong and frames can be produced cheaply and will not corrode. Its also very stiff. Some see this as a good point, but for me its a negative. I have an aluminium Gary Fisher frame which is 6 years old. Its been a great frame, but on any rough stuff if you try to stay seated its like being kicked up the arse.
Steel (my choice) can be lightweight and strong with just enough flex for a comfortable ride. It can have the potential to corrode however if not well looked after. I'm talking about high grade steel tubing by the way, like Reynolds 853 or similar. For me the major advantage of steel over titanium (other than cost) was the fact that it is repairable. If you break or ding a steel tube, a good custom frame maker can replace just the damaged tube, giving you a frame that can potentially last a lifetime.
Titanium on the other hand is the true uber-material for frame building. Its extremely strong, extremely light and a bloody comfortable ride. The only real drawback is the cost. And the fact it cannot really be repaired if damaged, but saying that at the cost of most Ti frames you can expect a decent warranty.
These are just my opinions, but to sum up, I personally would avoid aluminium bikes unless it was a full suspension, and would be happy with either a steel or titanium hardtail with geometry for an 80-100mm fork. This time round I've gone for steel, and aim to get at least 10 years riding out of it.