First you need to work out what your after?
Performance or Style
If its performance then you also need to ask yourself how loud you want your exhaust, if your happy to have a louder exhaust or not.
Magnex
Janspeed
Supersprint
Are all round good performers but more on the quiet side.
Piper
Scorpion
Mongoose
Are again good all round performers but with a slightly more agrresive exhaust note.
Ahsley
OMP
Are Very Good performers as they are more directed at competion and Fast Road and are very Loud.
But if your just lookin for a exhaust system that looks good and any performance gain is just an added bonus then you cant go wrong with a Peco system, huge options in regards to tailpipes where as the others your quite limited to either 3" or 4" single round.
Again Custom built exhaust may or maynot be best for performance but generally the people lookin for custom builds are more looking for that extra touch of style rather than a good performer - have it anywhere you want it in any shape or size.
Its a hard one to recommend because its as much a personal taste as any other item for your car!