Of the ones I saw, here's my top 5 .. but not necessarily in order of their ranking. I don't think I could rank them since they all have different styles. They all have different strengths and weaknesses.
* Lino Brocka. Of his films that I saw, I got blown away the most by Insiang. Not to underestimate his other works, I love most of them (Maynila Sa Kuko ng Liwanag, Tinimbang Ka Ngunit Kulang, Jaguar, Bona, Bayan Ko, Ina Ka Ng Anak Mo, Cain at Abel, Miguelito Ang Batang Rebelde) but his works from the late 80s onward were really not impressive (Macho Dancer, Orapronobis). My biggest "regret" if you can call it that way, was never seing Tubog Sa Ginto.
* Mike de Leon. When I saw Kakabakaba Ka Ba?, I was laughing so hard. I thought I should check out his other works. Kisapmata. Batch '81. Itim. Bayaning Third World. I think he also did Karnal, but I thought that movie was a little shaky.
* Ishmael Bernal. Himala and Manila By Night were good enough for me. Didn't he do Relasyon and Hinugot Sa Langit (that abortion movie by Maricel Soriano) ? I did like Nunal sa Tubig. I thought it was good story telling and the actors delivered good performances ( Daria Ramirez and Elizabeth Oropesa).
* Mario O' Hara. I saw Tatlong Taong Walang Diyos because of Nora Aunor. I was too young to care who the director was. Next time I saw it, I was already aware of these things and decided to check out his other works (Bulaklak ng City Jail, Condemned, Bagong Hari, which sadly didn't get a lot of attention as it was shown I think around the Edsa revolution).
I have a couple of names to complete my 5 best but I really can't decide which one to elevate into that pedestal.
Laurice Guillen - Salome. Init Sa Magdamag. If only she made more.
Celso Ad Castillo - Pagputi ng Uwak, Pag-itim ng Tagak, Burlesk Queen. I can't say much since I heard most of his best works were in the 70s, and sadly, didn't know any of them.
Peque Gallaga. Truth be told, I prefer his episode in the first Shake, Rattle and Roll movie, the one with Herbert Bautista and Irma Alegre (yata) in the Mananaggal episode over Scorpio Nights and Oro, Plata, Mata (great cinematography).
Maryo J. delos Reyes. Bagets. Saan Darating Ang Umaga? Tagos ng Dugo. All good films for me, but others I think were either too light or just didn't live up to my expectations.
I wish I saw more of Gerry de Leon, Eddie Romero, Chito Rono.