At the moment the best bike is one you can find. Demand has been high and supply constrained for a while.If you can, I advise renting a bike or two to see if you like mountain biking and what style suits you. Among newish bikes a 29" or 27.5" wheel hard tail with hydraulic brakes is a good starter and you can sometimes find extraordinary deals on old formerly high end 26" wheel bikes. Dual suspension is pricier but nice to have. Personally I like the older Kona hardtails and the 2008 and newer Trek Fuel EX series. Jamis bikes are also worth a look, both the hardtails and the Dakar series dualies.