Which wireless computer is best for mountain/trail bike?

Just about to start trail riding. Want to buy a cycle computer to track my progress. Any recommendations?

PS Got my bike, got my helmet
Can stay upright but can't go far YET!
Update: I'm just starting out!! Hurtling down mountains is a long way off!! Gentle rides along country trails is more like the thing. Just want to keep a track of how far I can go before I 'm beat...to track my progress
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