Which sclerotherapy solution works best?

No matter what sclerosant is used (hypertonic saline, sodium tetradecyl sulfate, polidocenol, or sodium morrhuate are the most common ones), it needs to get inside the spider vein to do its work! The smaller the spider vein, the harder it is to hit properly. Sclerotherapy, as you have found out, is highly operator-dependent, and less related to the solution used. All of the commercially-available sclerosants work well, but have different side effects, proper concentrations for uniform response, and different adherents among users. But all of them have one common denominator--they have to be skillfully and properly gotten into the vessel for it to "work." You have had poor results, I would submit, not because you have had a "wrong" solution (though to be truthful, it can be a partial factor), but because your injector simply missed the veins too often.

As someone who has performed sclerotherapy for 30 years, and taught laser surgery for over twenty, I can still unequivocally state that skillfully-performed sclerotherapy remains the "gold standard" for treatment of spider veins. Larger varicosities require other treatments, but it's the smaller ones that require the best technique and often are the biggest cosmetic concern. Lasers can work for some of the smallest spider veins, but usually at slightly higher risk of complications (blister, scar, pigmentation change, or inadequate result), and almost always at higher cost (It's the laser!).

Find your old sclerotherapist, or a new one with better skills. BTW, since you asked, I believe polidocanol is the best sclerosant, as it is the most pain-free, has the fewest side effects, and has great results when used in 1% concentrations with proper technique. I have use all of the others, so this is based on significant experience with these.

It can take up to 8 weeks to appreciate the results from your sclerotherapy treatment. Asclera and Sotradecol are the only 2 FDA approved agents for the treatment of small uncomplicated varicose veins of the legs.  Studies trying to compare the difference between these two agents and hypertonic saline have not shown a significant difference in effectiveness, but hypertonic saline tends to be more painful with injection and has a greater risk for ulcerations.  One must treat the underlying blue veins that feed the superficial spider veins, or treatment of the superficial spider veins will not be proven successful.  Results are dependent on the technique, compression, and sclerosant concentration used for the vein size being treated.

Sclerotherapy is usually done monthly for 3-6 treatments at a cost of $450 per session.  Usually Asclera is now the most recent solution FDA approved.  Whether by foam or by straight solution, both work very well.

Important considerations in sclerotherapy include the proper use of sclerosant solutions, but more importantly, the technique used for injection. Larger veins should be addressed prior to smaller end branches. As a fellow of the American College of Phlebology and a double board certified surgeon who is a Diplomate of the American Board of Phlebology and the American Board of Plastic Surgery, it is important for me to evaluate these venous patterns in a logical and comprehensive fashion prior to having treatment. Do not focus on the sclerosant as much as the surgical planning and knowledge of the treating physician.

Thank you for the question! Currently sclerotherapy is the treatment of choice when treating enlarged and bulging veins. In our practice, I use a sclerosing solution that causes the vein to shrink. Usually it takes only 1 to 2 sessions to achieve fading of the undesirable veins.  
An in-person exam with a board-certified plastic surgeon is the best way to assess your needs and obtain expert medical advice. Best of luck!