Hmm, this is basically a battle between the Vulpix/Growlithe/Ponyta lines and the Tepig/Litwick/Larvesta lines for me. Vulpix and Tepig are both very high among my top favorite Pokemon, and I probably give the edge to Vulpix. I'll take Volcarona over Rapidash, and... I think I like the Growlithe line a little more than Litwick's...
So I guess I'm Genwunning here.
Gen IX is probably third place? Fire mask is what I gave my Ogerpon, so that functionally was a Grass/Fire for me for the most part, and I love Ceruledge. For the most part, each gen only has one or two Fire types I really care about.