All 3 Packs are really great but if limited in cash and desire some LMP'S then get the GTE Endurance Pk.
With extra cash then also buy either one of the GT3 Packs according to what your car preferences are!
GTE Endurance Pack: BMW M8, Corvette C7R, Porsche 911 RSR, Norma LMP3, Oreca-07 LMP2
GT3 Challenger Pack: Aston Martin, Audi R8, BMW M6, McLaren 720S, Porsche 911
GT3 Power Pack: Bentley-Continental, Callaway-Corvette, McLaren 650S, Mercedes-AMG, Radical RXC