2-1 chrome slip on with Heat shields. Long to the collector, which means it will make more high RPM power at the lower RPM's expense. This is a "Diffuser Disc" system that is adjustable [ Back Pressure ] The Muffler section is a bit large and will scrape fairly easy. Some have complained that exhaust "Soots Up" their wheel/brake rotor. I like this pipe for high RPM power. You will need a PowerCommander or TFI [ Race Fueler ] or Race Tuner to make this pipe work.
+12 hp.

2-2 chrome slip-ons. I think they where first to market and are fairly popular [ in the early days ] They are very loud, even with Baffles, and have a very traditional sound. With some tuning they can make great bottom end power. They will not help on the top though and are more for show. They Need, PC3r or TFI or Race Tuner
-5 HP

A Very good chrome slip on, that disappoints most on sound [ or lack of , see V-MODse ] They make very good all around power and retain the stock look. You can use a HD download [ at dealer ] or PC3r or TFI box for this setup
+6-8 HP

A "Shot Gun" style of slip on that has a traditional look. Fair power gains and a almost "Drag Pipe" sound. Can use HD Download or PC3r or Race Tuner - (Note - these can also be VModed for enhanced sound as well as a additional pony or two)
+5-6 HP

2-2 SS system with "Mega Phone" Baffles. These about break even in power, but much louder than stock. They offer two baffle styles [ loud and louder ] they have a unique look and work well for our shorter legged V-rod friends. They require a PC3r or Race Tuner or TFI box.
-5 HP

2-2 chrome . About as close to a Big Twin sound as you are going to get. Loud and Proud, also good for Height challenged. They make great Bottom-Mid power, but flatten on the top [ too long ] Very good fit and finish. Baffles available, but still loud.
They need a PC3r or Race Tuner. To make these work you will need to mod the intake stacks and make timing changes.
+5-6 HP
 

2-1 Stainless This is a great pipe! I just wish they had Heat shields. The Stainless "wheats Up" [ yellows] immediately and detracts from the look, which is short and sweet, makes the bike look lighter. They run a small primary which works fine for STD bore, but is lacking for Big Bores.
Pc3r or Race Tuner is recommended
+10-12 HP

2-1 or slip ons, chrome . The 2-1 makes good power and has a great fit and finish. I never liked the "Note" of the exhaust. Just something I never got along with. D and D makes some huge power gain claims. I made gains, but not as large as they claimed.
You can use the HD download or Pc3r of Race Tuner or TFI box
+10 hp
The Slip-ons are very similar to the SE Slip-ons, A little louder but less power
+4-5 hp
 

2-1 steel, jet coat or black paint. This is a great race pipe. Great power increase on the top end. But I found a bit tough for the street. Some clearance issues [ minor] Can be a real leg burner. Very Loud, one of the loudest.
You will need a Race Tuner,TFI Box or PC3r
+12-14hp

2-1 steel jet coat or chrome. A very unique pipe. Utilizes a Force Air Induction Collector. Comes with a quiet Baffle [ which I like the best ] or you can run it open. Well made but, not cheap. A bit tough to tune [ on the low end ] but I believe those issues are being addressed. PC3r or Race Tuner.  +12-14 HP
 

Chrome slip on "shot Gun" style muffler. Almost the exact same as the SE 16GA , a little smaller Baffle then the 16GA, but a little louder then the stock 16GA with a little more power.
Need Powercommander, RT or 16GA download.
+8-10

2-1 slip on. Chrome. Very short to the collector, makes tons of bottom end with a nice increase up top. Its Loud, but deep. Really lightens the bike. Opens up that nice custom wheel you have back there.
PC3r or Race Tuner, TFI box
+6-7HP, huge mid range bump
 

Stock muffler modification. The way to go if you like the stock look. Great sound with a nice increase in power and a good 5 more ft lbs of torque in the middle.
You will need a PC3r or Race tuner can use HD download.
+6-8HP

A V-Mod'ed Screaming Eagle slip on. Very good power increase and a much better sound with out being too loud. One of my favorites
PC3r or HD download
+9-10hp
 

Chrome slip ons. Basically hollow pipes, very loud. Surprisingly, they work well. A unique "Turn Out" Look available in short or long. This will let them know your coming.
PC3r or Race Fueler, TFI box
+6-8 HP

Here is another option in exhaust that Steve did not comment on yet but has some potential as they are known for their high performance products.  More to come.....