i have to agree here, best is in the eye (or ear) of the beholder.
I bought Sils (see my other rants.. erm threads) and feel that the shorter can looks killer on high mounts. Frankly I much prefer the look of high mounts on the monster but thats me.
If I had to rank Sils on fit/finish, overall quality and installation I say they are among the worse.
Sound wise they sound great, at least the 105db models. A tad loud so I wouldnt want to go on a cross country trip but around town they are fun. I think almost any brand will improve the sound over stock pipes though.
Performance? Hard to tell, the pipes are lighter than stock so ANY weight saved improves performance that just wont show up on a dyno run. Midrange feels smoother but I think almost any less restrictive exhaust pipe would do this. Looking at dyno charts from other brands it appears to be the case.
If I had to do it over I might have gotten Termi's but for the same price Leo Vinci's look the best and sound great. Keep in mind performance wise they are all going to be somewhat the same to price is a important consideration but also look at fit/finish. You want a high quality product thats going to last. Finally find a retailer that is responsive and has a liberal return policy if there is a problem.