Holy grail: Not sure at the moment, pretty satisfied with what i've got already (near mint 8880, 8448, 8466, 8480, 8421 etc). An RC buggy set would be nice to make MOCs faster and more powerful.
Agreed. Some of the best sets are non-vehicle sets. Stuff like the X-Y plotter and robotic arm from Control Center 1, and how the B model of 8868 oscillates the pneumatic function, even though by modern standards their mechanics are rather simplistic. I also really enjoyed the days of the old un-geared motors that ran at multiple thousand rpm because they made for some impressive gear trains to drive mechanisms at low speed. I can see how it is not viable from a business point of view to make sets like that anymore though. The average 8-16 year old would rather a large truck to play with, even if it is mechanically uninspiring. I guess Dacta/Education and Mindstorms aspire to cover this area instead. Still, it would be nice to see TLG use the B-model instructions to create something wildly different instead of vehicle especially when the primary model is also a vehicle.
Regrets: Buying MISB medium size sets before collecting all the flagships - had to disassemble all the smaller sets to make room for the larger ones :p