I've tried a number of them and I prefer Cola de Caballo by Amazon Therapeutics Laboratory liquid alcohol extract. This is their silica product, an extract of horsetail. It seems the most effective and easiest to assimilate. I get it from Vitacost. I would not order it directly from Amazon Therapeutics, based on my experience with that. Most of their products are very vital and potent, and are wildcrafted in remote areas, but l think they are using herbal products themselves, if you know what I mean.
I also tried Jarrow's Jarrosil. I like Jarrow but this didn't seem to be an organic product and wasn't as potent as the Amazon one. I tried Herb Pharms liquid alcohol horsetail extract. Like other Herb Pharm's products it had a weird color and taste, kind of like wood or tannins. I just don't like their products. It wasn't as potent. I tried Nature's Way horsetail capsules. They seemed to be somewhat hard to digest, for me. I generally like that brand and I believe that their products are fresh, good quality and as labeled, but not everyone can absorb every herb in that form (ground up raw in capsules).