Cleaning is as important as sanitizing. Without a debris-free surface a sanitizer loses most of its efficacy. This is worse for some than others, StarSan supposedly retains more sanitizing power in the presence of organic material than Iodophor for example, but it is never ideal. While scrubbing with a mesh pad to remove krausen and residue is fine for stainless steel and glass, the less abrasion applied to plastic the better because scratches provide shelter for microbes. The best solution is a soak or recirculate hot water laced with cleaner. Commercial breweries use caustic, but the care required for safe handling means it generally isn't used at home (although it is available as line cleaner). Most homebrewers use longer contact with milder alkali percarbonate cleaners like Five Star PBW (.75-1.25 oz per gallon - $6.75/lb) or OxiClean Free (1-2 oz per gallon - $3.66/lb) for their fermentors, gear, and kegs.
I had a bunch of dirty one-gallon jugs from splitting five gallons of wort nine ways (three yeast each fermented at three starting pH values) for my December BYO Advanced Brewing article (subscribe) Acid Tolerance of Brewer's Yeast. I realized I had four alternative cleaning products on hand. I took five of the jugs, dumped out the beer, and let them dry for three days to give the cleaners a real challenge.
All prices listed are for "reasonable" 3-5 lbs containers (most links support the blog). Although suggested concentrations vary (listed), I used 1 fl oz per gallon (8 ml per L) for all for a fair comparison.
For a control I filled one jug with hot (110F/43C) tap water water. Even after a week it really didn't seem any cleaner. The krausen ring was still almost completely intact.
Seventh Generation Free and Clean Natural (~.3 oz per gallon - $4.84/lb)
The 2X version is suggested by BetterBottle at a rate of 1 oz in 6 L. It is enzyme-based and won't degraded plastics like long-exposure to alkali cleaners can. While it removed most of the krausen ring, some remained even after a week (pictured). I wasn't even able to rinse the rest off with hot water, it required another soak with a different cleaner. It might do a fine job on fresh krausen, but even with triple the suggested usage it was the worst performer of the cleaners.
Craft Meister Alkaline Brewery Wash (1-2 oz/gallon - $5.70/lb)
This is a PBW competitor manufactured for brewing. It did a good job, with the sides looking completely clean after 12 hours without any scrubbing or agitation. I don't care for their packaging though (it leaked in transit, and doesn't seal tightly enough to prevent moisture ingress during storage).
Blu Aktiv Brewery Cleaner (1-2 oz/gallon - $8.25/lb)
I've been using this for about a year after the company sent me a sample of their more eco-friendly (no EDTA or NTA, low phosphate) brewery cleaner. Luckily it still performs admirably, and cleaned the fermentors in similar time to the Craft Meister. However, for me the greenness likely won't be enough to justify the added expense and effort to procure once my supply is depleted.
Logic One Step (.5 oz/gallon - $6.00/lb)
I used OneStep on my first two batches as both cleaner and sanitizer. While it isn't certified as a sanitizer, there are many brewers who use it like one with good results. The surprise was that it removed the fermentation residue in about six hours, beating the two specialized cleaners! While it is twice as expensive as OxiClean Free, the suggested rate is considerably lower.