I'm personally more into vintage, classic, bluesy material so by category I'd pick TF Digital Delay, Vintage Delay, Tape Delay, Filter Pong and of course the Looper (but also ModDelay, Reverse, and MultiTap if you are creative.) MF Chorus, Wah-wah, Rotary and Tremolo are mandatory; Phaser and Flanger if you are heavy, Undulator and RingMod if you are wild. PF PitchFlex and MicroPitch are incredibly useful even for tame stuff, all the pitch shift algos are great if you are progressive. From SPACE if you love reverb you need Hall, Plate, Spring, ModEchoVerb, Shimmer, and BlackHole for sure; MangledVerb to probe the dark side. H9 Ultratap is a great modulated multi-tap delay with chopper tremolo, and the Resonator is a powerful special effect with selected pitches ringing on delay taps.
