Horus, Bastet, Dark Metatron, Anubis, Ra, Haku, Red Sonia, Arch Angel Lucifer.
Arch Angel Lucifer, Bastet, Red Sonia, Dark Metatron, Haku, Horus, Anubis, Ra. If you get ANY of those, keep the account. I listed them in "ease of play".
Lucifer is the worst tank and grind part of the game, but trivializes content that can be otherwise hard.
Bastet is (eventually) a good 16x lead, with a scaling damage modifier based on the amount of combos.
Red Sonia is a rare roll that should allow you to clear through some content pretty easily.
Dark Metatron is heavily geared toward specific team members that you get through in-app purchases, but can be damn powerful.
Haku is a great spike lead, easy to level, and not too difficult as a lead.
Horus is probably one of the best spike leads, and you don't need to evolve him to actually use him. One of the best available.
Anubis is hard to use (10 combos, aka a whole board, to get 10x multiplier), but is the end-all-be-all for spike damage. New form means you can do 8, 9, or 10 combos for 4x, 7x or 10x (or 10 - 15 combos for 10-15x). Can be a game changer.
Ra is crazy. Match one of each color and get 6x. New form is match 4 different colors and heart or 5 different colors for 4x, match 5 different colors and hearts for 7x.
I also think Isis can be decent (especially with her new form) as well as Kirin and/or Umisachi&Yamasachi as well.
Vorph is right, start as soon as the Dark festival starts (Friday Morning). Festivals are weeklong, and give a 1.5x increase for the specificed monsters. Godfests are 24 hours and give a 3x increase. If you get Haku or Anubis (or Archangel Lucifer), start clearing dungeons as fast as you can for free stones and roll on godfest as well.
There's like 0.58% chance of rolling a specific monster in the REM. It's 0.84% for a festival, 1.74% during godfest, and 2.61% during a godfest+carnival. That's only increases for being featured, as I'm pretty sure the chance for a non-godfest/non-carnival monster to appear goes down, but I have no idea.
So, don't really try to roll for specific gods (unless you're rich or rerolling).