Leslie is one of my favorite effects. When I was a kid I used to love to hear the big Hammond organs play through a full-sized Leslie. Of course, Leslies were fairly common on recordings of guitar in those days, and I always wanted to have one for my guitar. I got lucky a couple years ago and bought a Leslie 147 with the combo preamp.
Accurate emulation of a Leslie is extremely difficult. None of the emulators can stand up to a side-by-side comparison with a real Leslie. There are a few, however, that get close. I'm going to restrict my remarks to those emulators I've actually played and are still in current production.
- DLS RotoSIM: This is best of the emulators, IMO. They got everything right - the horn/rotor independent speeds and accellerations, crossover frequency, power-amp grind, even the cabinet resonance. This is as close to the real deal as you can get without lugging a hundred-pound cabinet. You can pick one up used for about $200 to $225.
- The Leslie mode of the Digitech EF-7: This gets an honorable mention for getting the basic sound and horn/rotor behavior correct. They missed some of the details that DLS captured, and the treadle control just isn't the same if you're used to a traditional fast/slow stomp switch. There's a small but noticeable effect on the bypassed tone. At $200 new, this might be worth consideration if you had an interest in some of the other effects this pedal can do. But for just Leslie I'd go with the RotoSIM.
- The Boss RT-20: This pedal is a distant third. There is a decent Leslie emulation in there, but it's hard to find. Like most Boss pedals the controls allow you to dial in a lot of over-the-top sounds. If you know what a Leslie is supposed to sound like you can find a setting of the RT-20 that gets pretty close. But it's a narrow range that yeilds an authentic Leslie sound; the rest of the range gives all sorts of odd mutuant Leslie variations. The overdrive control is a joke; it sounds like a distortion stompbox rather than the grind of a Leslie power amp.
Those three are the best (with qualifications as noted) of the current crop of available emulators. There are two more still in production that are worth a mention. The H&K Rotosphere is highly regarded by others, but never really convinced me of its veracity as an emulator. The Voce Spin II is quite convincing, but is a bit touchy about input levels.