TMC and Maruyama are a couple of the brands most used by the pros. I once found a Maruyama that a lady had put on the street. She said her dad died and she had a month to clear out the house and didn't want to be bother with selling stuff. I brought it to the local japanese lawn mower shop to get a head for it and mentioned to the owner that I found it and wanted to sell it. He wanted it. After buying it, one of his regular customers (a professional gardener) came in and he was soooo thrilled to tell of his new purchase. He said, "It's a Maruyama! I'm taking this home! I could never afford a brand new one of these, now I've got one!" This was coming from the Japanese store owner/repairman who's been there as long as I can remember. Judging by his excitement, I'd say it was a top of the line trimmer. The gardener mentioned he had the same thing. These are both commercial brands, though, and may be more than you're looking for or need. (performance and PRICE wise (they're not cheap!)) Also, you're probably not going to find these two brands at your local home and hardware store. Makita might be ok, too. After my Grandpa died, I sold his Makita string trimmer to a professional gardener. The gardener told me he actually used a TMC, but needed a new one and didn't have the money for it (he said his cost $600), so he just wanted something cheap for the time being. I sold it to him for $100. He didn't even start it, just lightly pulled the starter cord to listen to the "put" sound it makes when slowly pulled and just said, "ok, I'll give you $100 for it." (my asking price in the Recycler)
The gardeners that do my parents house also use TMC string trimmers. They do many houses on our block, going from house to house, and when they're done with our neighborhood, they move onto the next. They're running their trimmers everyday, all day. If they're chosing TMC, I'd say it's a good bet that they're good tools.