The Fusion Calculator is daunting at first but can be helpful if you want certain skills.

Earlygame:

Shikigami & Lilim = Take-Minakata: Has solid offensive stats for earlygame, Repel Elec is real nice in Ikebukuro and Kabukicho where there's roughly ten million enemies with Mazio, and early source of Focus for physical fusions down the road. If you can stomach the initial trouble of leveling up Shikigami four times for Tarukaja, you'll coast with Take as your baseline. Take & Mofunofu = Naga who basically does the same job as Take with way more HP, and eventually he evolves into Raja Naga for even more bulk.

Datsue-Ba & Hua Po = Bicorn: Learns Sukukaja at base, which is near-mandatory against Matador. Can potentially pick up Rakukaja from Hua Po as well. Sudama learns it too and you can meet it before Matador, but its vitality is horrendous. Best not leave anything to chance against Spanish calcium man.

Kodama & Apsaras/Isora = Inugami: Early Media Fog Breath, can evolve into Makami who gets Null Mind and Diarama. Level up Kodama once to pass Dia for earlygame healing.