Attenborough will always be number one. Not just because he's brilliant at it, but because we've all grown up with it being him narrating on some of the most amazing and mind blowing natural history films ever made. The big man's voice has been described as 'soothing', with pretty much a throw back neutral 'BBC' accent, and he speaks with authority but without being intimidating.
In terms of who could fill his shoes..., loads have had a go, including the likes of the already mentioned Ewan McGregor (Obi Wan Kenobi), David Tennant (Dr Who), Pierce Brosnan (Jim Bond), Andrew Sachs (Que?),