Interesting question, but difficult to answer. This is like asking "What's your favorite [movie, book, song, ... ]. It depends on many things, including your mood and the task at hand.
There are many variables to consider with keycaps:
+ Switch/stem type, e.g., Topre, Alps, IBM BS, Cherry, other.
+ Vintage or contemporary keyboard.
+ Keyboard form factor.
+ Keyboard chassis.
+ Keyboard layout, e.g., ANSI, ISO, HHKB, Dvorak, Colemak, other.
+ Keycap material, e.g., ABS, PBT, other.
+ Keycap profile, e.g., Topre, IBM BS, Cherry, Leopold, OEM, DSA, SA, DCS, other.
+ Keycap tops, e.g., Spherical, cylindrical, flat, other.
+ Blank, legended, or relegendable.
+ If legended, legend type, e.g., Double- or tripleshot, dye-sublimated, shine-through (backlit).
+ If legended, legend position, e.g., Upper left, centered, lower left, front, other.
+ If legended, font and typeface, e.g., Helvetica regular, Helvetica italic, other.
+ Keycap color scheme.
+ Keycap feel (in part a function of material).
+ Keycap thickness.
+ Keycap sound (a complex function of many variables).
+ Purpose (e.g., aethetic, functional).
Although I like the appearance of some vintage spherical thick doubleshot ABS found on a variety of terminal keyboards, this design is not practical for me.
Instead, I try for the best combination of keycap variables to suit a given keyboard form factor, layout, and switch type. Generally, I tend to prefer dye-sublimated PBT cylindricals in a sculpted profile. The profile can be afforded by the keycap set or by the curvature of the board, as with IBM BS.