Electric: I'd definitely go with Electrode. His defense stinks, but his speed more than makes up for it in my eyes. :D
Grass: Believe it or not, one of my favorites is Tropius. I just think it's cool, I guess, but if you go with that one watch out for Ice attacks! XD
Rock: It's hard to find a rock Pokemon that I like. They're all too slow for me. But if I had to choose one, It'd be Relicanth. I like the combination of rock and water. Watch out for grass types, tho!
Steel: I like Magneton ok if it has the levitate ability. Of course, Metagross rocks too. Its resistance to fighting types makes it a hardier one than Magneton. :P
Fire: I always want Blaziken if I can get it, but if not, I'd go with a Rapidash. Is that even in Ruby, Sapphire? Err... or maybe an Arcanine?