Aside from the obvious bias towards Typlosion, Entei and Ho-Oh, I'll go with the Magcargo line, Houndoom line and Magby.
I know I've covered the entirety of Johto's fire types, but I like em all. Magmar was always one of my favourite Fire types in Kanto, so although weak in battle, Magbys design is superb. Houndoom, from what we see from Johto's Rocket Leader and Karen, Houndoom is a force to be reckoned with and wish they allow the opportunity to catch Houndour early on, like around Goldenrod, for instance. There were so few additions to Gen2 in terms of fire types, Slugma just happened to fall right in.