LensRentals.com - Good Times with Bad Filters. Is a great example of how filters affect image quality. Most folks I know don't put a filter on top of the lense. I put them on all of mine because one scratch and your \$s lense is well not so nice.
In terms of brands, I've been using B+W (very, very expensive) and not Heliopan (even more expensive). But fredmiranda.com points out there is a review of UV filters. Normally you put a UV filter on as protection from http://www.lenstip.com which does a review of UV filters. I had not seen that before and surprisingly they like the Hoya filters which is a huge surprise they absolutely disliked the Tiffen product.
They apparently don't make the UV-0 series anymore though. And from the chart you can see there is definitely a difference between the more expensive pro filters and their other lines. At the time of this review, they were the Hoya UV-o and the B+W MRC.
Today, the latest versions are the Hoya HD and the B+W XS-Pro and stackexchange.com that they are both great with the only issue being Hoya uses aluminum whereas B-W uses brass says. And dpreview.com explains the new HD2 carries a longer warranty. It is UV and Protector filters are lifetime and others are five year.

 Filter Result Result [%] Price[PLN] 1 Hoya 72 mm HMC UV-0 36 90 79 Test Hoya 72 mm Pro1 Digital MC UV-0 36 90 155 Test 2 Hoya 72 mm HMC Super UV-0 35 88 109 Test 3 B+W 72 mm 010M UV-Haze MRC 33 83 190 Test 4 Hama 72 mm UV HTMC 390 (0-Haze) 32.5 81 139 Test 5 B+W 72 mm 010M UV-Haze MRC Slim 32 80 325 Test Marumi 72 mm MC UV 32 80 128 Test 6 Sigma 72 mm DG UV 30 75 229 Test * Kenko 67 mm Pro1 Digital 30 75 149 Test 7 Heliopan ES 72 mm UV-0 SH-PMC 29 73 271 Test 8 Marumi 77 mm WPC UV 28 70 151 Test 9 Hoya 72 mm UV-0 - Standard 26 65 79 Test 10 Hama 72 mm UV 390 (0-Haze) 24 60 109 Test 11 B+W 72 mm 010 UV-Haze 23 58 120 Test B+W 72 mm 010 UV-Haze Slim 23 58 210 Test 12 Heliopan ES 72 mm UV-0 21 53 182 Test 13 Marumi 72 mm UV 20.5 51 91 Test 14 Hoya 72 mm UV G - Series 11.5 29 39 Test 15 King 72 mm UV 10.5 26 59 Test 16 Tiffen 72 mm UV 3.5 9 72 Test