Up until recently I would have said Chomp but now its Handcent. The simple reason being that the latest Chomp update is a disaster on my HTC Hero. It force closes when you try to send an sms. It doesn't receive sms's any more. But what it does really well is plaster istelf with adverts!.
I uninstalled chomp - installed handsent and got a flood of sms's coming through which chomp had managed to block.
Well done chomp!