Valhallan legs!

Chain weapon pack, Force weapon pack, probably any other weapon packs would be nice. I think I had the melta one somewhere but that's about it, and those have limited uses really, what with the IC rarity factor and being a bit overpowered for the average game... Las-weapons pack would obviously be the holy grail, but
no one has those so far as I know (I think there's a thread somewhere on how that tragedy occurred). Bloody Paulus is a nice thought; I don't think I've ever seen a painted Paulus in the flesh. Moulds of all the servo-skull variants would probably come in handy too.
Re: full models as opposed to conversion packs, in an ideal world I'd vote for Scarn and Sabellus/Crime Boss if you have either, followed by any/all Imperial civilians you can get your hands on, and maybe also the Sister Repentia. Unbound Daemonhost and Callidus Assassin are probably equally rare, but I doubt there's quite as much demand for them.
By way of a wild card, if you have the Artel W Witch Hunter that'd be an amazing non-GW choice.
In theory I'd buy recasts of any of this stuff, just to have it "in the bank" for when I get around to modelling again.