I don't really like to rank by producer since scenario writers, artists, and composers tend to move between companies quite often, nevertheless my top 10 in no particular order would be:
light - I don't like most of their chuuni stuff but they do have a couple decent ones, my favorites being 早狩武志のひこうき (gunjou), 潮風海, RURUR, and Vermillion.
flat - rising company, notable works are Secret Game 1, 2, and last year's popular ひこうき雲の向こう側.
5pb - The entire memories off series, and please port them to pc.
nitro+ - Muramasa goes without saying, along with Jingai Makyou, Kimi to Kanojo to Kanojo no Koi, Phenomeno, and Gekkou no Carneval.
leaf - the classics To Heart 1 and 2, White Album 1 and 2.
Lump of Sugar - now called lump of shit, but they made tayutama and itsusora.
Akatsuki Works - the classic Rui wa tomo wo yobu, then Devils Devel Concept, and the more recent Hello Lady for action VNs.
GIGA - この青空に約束を―, パルフェ, and the BSD series.
Crossnet/Favorite - not that good but will sell because Shida's followers.
Saga Planets - some of the better slice of life out there. Hatsuyuki Sakura, Gold Star, and Natsuyume Nagisa.
Navel - Shuffle, Soul Link, Oretsuba, and Tsuriotsu