Khador and Cygnar both seam like the heavy hitters jack-style if you ask me. They both have the largest range of big jacks too, and with MKII rules, they encourage more jack-style play too boot, making either the ideal choice. Both I think have the coolest looking jacks too out of them all.
I too play to have fun, and like to use the models that I prefer the most, versus having the absolute best competitive army.