It depends whether I'm going to be out in it for 1 hour or 10 hours. It depends on how warm or cold the days is and what kind of riding I'm doing. Often MTB is so high intensity you boil in a hardshell. As an all weater all year commuter my wet weather wardrobe is pretty extensive:
Gill eVent cycling cut 'traditonal' hardshell – totally waterproof, but even though its fancy eVent, it still gets pretty hot and wet inside if you're going hard. It also feels horrible to wear, so I hardly ever wear it, unless I'm out in the cold at low intentsity in stairrod rain all day.
Various softshells – these I find much more versatile. Not completely waterproof but they'll shrug off decent showers and remain warm when wet. The do feel brilliant to wear and are much much much more breathable then the hardshell. I have a bright orange howies cross that I commute and winter train in, it gets worn pretty much every spring, autumn and winter day. Microfleece lined so too warm to wear in summer downpours. Happy to wear it for 30 mins to all day. I have a grey arcteryx with a very very thin quatered microfleece lined, which is a bit cold to stand a round in, but much less hot to wear, it gets used for MTB when the weather is changible. It'll take a biblical downpour to make we use the hardshell over this.
Montaine Superfly: Basically an emergancy windshell that'll shrug the worst off if I get caught out. Packs down to the size of an eating apple so very easy to take with your anywhere.
Water resistant cannondale roubaix biblongs. A complete PITA to get into and hardly very comfy on the bike either. Only worn if going out on the road in stairrod rain all day.
Cheap plastic walking trousers cut off at knee. Good for MTB in the wet, stops you sluicing your buttcrack with gritty water all day. You slide around a bit on your saddle but its worth it.
Winter boots – best invention ever.
Merino socks – wet, yet warm.
Windstopper gloves – waterproof gloves are awful to wear, so I tend to allow my hands to get wet, but windstopper keeps them warm enough.
Insulated waterproof gloves. A complete PITA and like trying to operate a nuclear reactor control desk in oven gloves, but essential if its sub zero and very wet.
Silk glove liners. Give you a few extra degrees warmth in nasty weather if wearing std gloves becuase control is paramount.
Its pretty much impossible to stay dry on the bike, so I tend to wear whatever will shrug the worst off and keep me warm to the end of the ride.