This has got to be one of the toughest questions... I thought about this but there are so many great stamps and I like many for different reasons. . Some of this might sound a bit corny... I collect US, so... I don't think I've been happier with a purchase than when I got SC#1, obviously because it was our first regular issue. A bit of National Pride I suppose.

SC# 771/CE1 because I like the design and I love the seal. National Pride Again!

SC# 292 because it's beautiful and I see strength and duty when I look at the artwork: "The vanguard" by the Scottish artist MacWhirter.

Series of 1902 (#300-315) because I like the designs and I do tend to gravitate toward engraved designs.

But then the back of the book offers a lot, particularly in the Private Dies designs...
However the ones I covet the most lately are the Embossed Revenue Stamped Paper, British tax stamps RM15 through RM35, again it's that National Pride thing... (No offense to the Brits reading this.)

Sorry I just can't get it down to one.
That's OK, I can't really pick a favorite single movie, song, food, etc. either. Except maybe bands... I'd have to say Pink Floyd wins overall but even that depends on my mood. In the end, we're each 'tainted' by our own experiences in life.
Just my humble opinion.