Tates are clearly the best, but they're not cheap. $400 + these days. I was in the business in the Quad heyday and beyond. In my opinion, most SQ circuitry is poor, including nearly everything built in to a receiver. Although I intend to upgrade to a Tate one of these days, I own the first three decoders listed and would highly recommend any of them.
1-Sony SQD-2020 was one of the first quality SQ decoders. It featured full-logic wavematching, the most advanced for its 1973 vintage. It can also be used as a Quad preamp. including to convert a stereo receiver to Quad. You would still need an amp for the rear channels in using it with stereo receiver/amp. If you're a gearhead, as I am, you can never have enough inputs. This box will allow for expansion and control of a Quad system. It also has an RM decoding position, which I've found to be adequate for QS decoding & synthesis. I could not find current listings on E-Bay, Craig's List, or Amazon for this piece. A few years ago, I'd seen them for $100-200. $229.95 was the original list price.
http://www.thevintageknob.org/sony-SQD-2020.html IMHO, SQD-2020 and 4-DD5 are the two boxes from the heyday that will allow you both flexibility and decoding/demodulating. I've owned them both for 40 years.
2-Sony SQD-2010 was similar to SQD-2020, with a different chipset. It has nothing but an on/off switch and is STRICTLY for SQ decoding, not QS or synthesizing. I couldn't find current pricing, but I'd expect the cost to be 1/2 of what the SQD-2020 costs, proportionately the same as when they were new.
3-If you have Marantz Quad gear, you have an expansion slot which will fit their SQA-2 and 2B modules, which are their full-logic decoders. I own an SQA-1 front-back logic decoder, which I retired when I bought the SQA-2B. SQA-2/2B have similar performance to the Sonys above.
4-The Lafayette SQ-L is not full-logic, as the others I've listed, but the results are surprisingly good. This box can give many full logic decoders a run for their money.
5-Lafayette later made an SQ-W full logic wavematching, which is listed at $66 today. It has 2 synthesizer circuits. A good value.
http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=260990128548+&item=260990128548
6-Lafayette built some great decoders into their receivers, too. Their LR-4000 was highly rated in the day, and is still viewed as a high quality, full-logic wavematching decoder.
Another question.
What is a relatively inexpensive but quality SQ decoder? Along the same lines at the JVC 4dd5.