Hmmm, I have to think about that. Right now I'm going to say my Thinline half pad. I got it for a great price on Ebay as a "second" from Thinline which apparently means perfect except maybe an uneven edge? I noticed an immediate difference in Annie's way of going when I added under my already well fitting saddle and an immediate and grumpy difference when I left it off one day (she literally would not move). I'm not a trend person, or a gimmick person but I really believe that it makes a difference in my horse's way of going. It's a must have for me.
What I'm lusting after right now is a Synergist endurance saddle. I'd love a comfortable and lightweight trail saddle and these are NICE saddles, custom made to your exact measurements and custom fit to your horse. Enough leather to keep you comfortable and in the saddle but not enough leather to add weight. Lovely saddles. It's a lust because it's way out of my price range and I don't really trail ride enough to warrant owning one, but one can dream.