Biggest disappointment has to be my old Trek Fuel Ex 5, 2007 model with the older 100mm travel suspension. It was heavy, slow and the suspension was constantly pumping making every climb a pain. Sold it and got a HT. It was almost new when I bought it and I was expecting much more from it after replacing my old Scott G-Zero. But it was heavier and felt much slower. The newer HiLo suspension Top Fuel I have now is much better, probably even better then the NRS described below.
Biggest surprise was my NRS, got the frame cheap from here (as always wanted to try one) and I was not expecting much from it. It turned out to be super efficient on climbs and with a newer shock improved the suspension even more, very quick and efficient bike for not much money.
Next surprise was a Kinesis Maxlight XC, great handling xc bike that was reasonably light, while was not as bad on my back as the older Giant ATX I had.