Aggresion Factor, Attempted Steal %, Flop aggresion, and 3 bet %. I think those should help us out, also how loose the player is. I may be wrong on this, so ask few other people.
To the point i would not even think of playing without it.
HUD is a huge help IF you know how to use it. Basically it helps you to better define villain's range of possible hands. So it helps a lot if you can think in terms of ranges properly, and it helps not at all if you can't.
That ^^^, it helps me figure out my opponents range, also I like to use the Fold on the flop %, sorry this has nothing to do with bluffing, but if you play cash games, it really helps, like for example yesterday I was playing on my private table, there was a guy with a really high fold on flop % because he didn't know how to play unimproved and yet he was very loose, so I kept constantly attacking, and I made more by winning small pots then he by winning one big one.