I'm going to chime in to agree with Jeff... the FM7 & FM8 are far closer to their hardware cousins than pretty much any emulation I've tried! I still have a pair of TX-81Zs in the rack, I like the way they react to guitar controllers. It requires some effort, but you can import the TX-81Z patches into FM7/8.
Some of the Korg digital emulations are pretty cool, Wavestation is one of my favorites.
I love the NI Prophet, but it does not sound like any P5 I played. Same goes for the Korg MS-20, I still have that hardware, and still use it. The thing about the software MS-20 is that it'll do things the hardware can't.
The Wayoutware Arp 2600 emulation comes as close as anything to the original hardware - and that's a much more difficult feat than emulating something that started out as software - Jeff's point about the DX7 being software is very important!
Bitflipper is right to a point, the poor DX7 was over-used, over-exposed even. It came along with a feature set that really was unbeatable. Thus everyone had to have one (I used to pride myself on never owning a 'real' DX7, right up until I bought a TX7, and later a TX816<G>). But in spite of the ubiquitous patches overrunning the airwaves, it can do a whole lot more.
And Jeff, that's a very generous offer, look for a PM!