Modern era and HC only, it's a toss-up between Frank and Bo.
Frank had early success but the farther away he got from the TO era the more things unraveled. Bo's teams were consistently good, just never great. I'll give a slight edge to Frank. At the very least, he proved he could reset things in 2003 with a staff shakeup which proved successful over 2002.
I think we all know who the worst is.