Best:
1. Yoga Mat - I currently have a Harmony which is nice but is sometimes a little too sticky for my taste, and despite having it for 6 months, I cannot get the smell out of it.
However, I'm not a yoga mat snob. I normally do yoga in my bedroom where my Harmony mat is, but on the occasion DH is using the bedroom to sleep or whatever, I'll do yoga in my basement w/o area, and I have no problems using my old cheapy mat from Target.
2. TransFirmer - I wouldn't do a Firm video to save my soul, but I use this thing almost every single day as a weight bench. I'm really using it on my current BBC rotation, and I use it for Cathes and anything requiring a bench. In a moment of lazyness I'll use it as a step too - two different heights! Although when I'm doing a pure step w/o, I'll use my regular step.
3. Polar F11 Heart Rate Monitor - Don't know how I ever got along without. With my OCD nature, I can't believe I survived just "guesstimating" the intensity at which I was working out.
4. Dumbbells - I have acquired a lot of them, so I have just about every weight I need (although I sometimes have to juggle two weights in one hand). I'm slowly acquiring every weight that I need. These too get used almost every single day.
5. Rebounder - I have a really old one, back from when they were simply called mini trampolines. Cast off from my in-laws, which they actually gave us because they thought the kids would like it (and to get rid of it for free). Considering I have PF, without it I would never be able to do a lot of the high impact cardio stuff that I've grown to love.
Worst:
You know what? Other than some dud fitness DVDs, I can't really think of anything I've bought that I don't like. These items don't get used as often as the others, but I don't think I'd get rid of them: stability ball, gliding disks, pull-up bar, multiple length resistance bands/tubing, Magic Circle, weighted gloves, ankle weights, Danskin balls in 3 and 5-lb weights, Firm Sculpting Stick (I actually will use this from time to time (usually for iBallet w/os), but as has been stated, the "twigs" are pretty much useless).