My husband is UK citizen and I have UK spouse visa. Thanks to my husband's job we would like to take one European trip a month. To that end I would like to know which embassy will issue the longest validity Schengen visa. I have already heard that the French embassy gives a multiple entry visa with at least 90 days validity. Can you suggest any others?
90 days (in any 180-day period) is in any event the longest you can stay on a Schengen visa. Schengen visas can be valid for up to 5 years but this has no bearing on the maximum stay, which is still at most 90 days. After that, it's usually necessary to leave the Schengen area for another 90 days before being allowed to return and use your visa again (at least for those who don't have an EU citizen spouse, more on this below).
To stay longer than 90 days at once, you would generally need a national long-stay visa or residence permit, which would allow you to take up residence in one specific member state but would not elsewhere in the Schengen area. If you are travelling/living with your British husband, the rules are however a bit different and you would be entitled to such a residence permit (see this question on another site for a discussion of the finer points of these freedom of movement rules).
Beyond that, where you should apply is not up to you. Except if you change your plans accordingly or simply lie on the application form (a bad idea), the consulate you should apply to depends on where you want to go. So if France is your main destination, you should apply to the French consulate, period.