The first thing to say is that there is no perfect MTB! Your perfect MTB depends on two main factors. What you intend riding on and how much you are willing to spend.
The three main MTB catagories are...
Hard Tail
Hard Tails are a lightweight, fully rigid rear end bike used throughtout all aplications of off road riding. If you're just starting out on your MTB adventure with function over finance being a factor, Hard Tails are where you should be. Hard tail bikes will generally have increased componant spec than equally priced Full Suspension bikes and, crucially, weigh less. A big factor to consider as weight saving on Full Suspension bikes comes at a cost.
Full Suspension
Does exactly what is says on the tin. Front suspended forks with a single rear suspension dampening system. Full Suss bikes offer a smoother, more controlled ride and will be needed for advanced trail terrain. Advantages to a nicely spec'd Full Suss over a hard tail bike would be higher downhill speed with improved control, less fatique on long, hard rides, oh, and they're great fun! As you look along the price range you will find the quality improve with weight saving top of the list along with fork lock outs, Hydraulic Brakes, adjustable dampening and suspension travel along with pre load adjustment. Sub Catagories which harbour mainly at the Full Suspension dock are Enduro, Trail Riding, Dirt Jumping, All Mountain and Downhill.
Rigid
Rigid bikes are found aimed largly at the leisure and urban sector, marketed at the entry level of MTB's and also the higher boutique end. Often spec'd with thinner road tyres more suited for less aggressive surroundings. The Hybrid bike and Rigid MTB share alot of features within their own definitions.
26 / 27+ / 29er
Back in the late seventies, when Tom Ritchey and Gary Fisher literally built the first MBT's 26" wheels were what took the bikes down the californian hillsides. Taken from the beach cruisers of LA, the 26" wheel stuck, for years. Today the industry standards are the original 26", the 27+ (27.5") and the 29er (29"). The 26" lends itself nicely to a lighter, more rigid feel with better acceleration on the hard stuff. The 29er is the direct opposite in that they feel heavier and harder to get going. The plus point to these larger rims is traction with slicker tyres and speed when your negociating trails. Harness that with a smoother, more stable ride.
Mountain Bikes