I've tried almost every other, except NaviCat. This is by far the best. Both MySQL and PostgreSQL have great CLI clients, but when dealing with big tables it is sometimes difficult to browse results (especially with large text fields/ image blobs/ etc.). That's where a GUI makes itself useful. A GUI client at a minimum needs to:

(a) hassle-free/easy manual SQL composition and execution (don't need to lock oneself into idiot mode all the time)
(b) easy way to view, browse, sort and edit large tables and query results
(c) form-based editing of records with large/expanded text fields
(d) code-completion (to avoid having to remember all those column/field names!).

RazorSQL does all this (and much, much, much more). It's SQL editor is amazing. I could not bring myself to try NaviCat because it seems silly to have to buy separate GUI's for MySQL and PostgrSQL, since all that's involved is plugging in a new module to the abstraction layers, but of all the other Mac-based SQL front-ends I've tried, RazorSQL has stood out clearly as the best.