These foul and winged girlies took me quite a while to glue together even though the kit-bash was very simple indeed: Tyranid Gargoyle wings grafted onto Daemonettes. I know I'm not the first person to try this, but I think my early results will eventually produce some of the very best examples. I've gone for dynamism over the practicality of making certain they rank-up on the tabletop... I'll deal with that sometime later.
I'd never suggest for a moment that there was a best way to do a particular conversion - personal preferences certainly rule - but on a trawl round the web I've found a few kit-bash conversions using some very odd choices (in my opinion); ungors for example, and even crypt ghouls. I can't see how anyone wouldn't opt for daemonettes, or, why they would add the extra arms at the front (like the meat-puppet-handed old sculpts GW still offer). In my mind harpies look something like this, even if mine aren't so feathery!