Candidates for best squirtle (w/o breeding obviously) below, frame x on tid y is just the closest, obviously all are hittable from any tid if you wait up to ~2.3 years:

rash:
frame 199499195, stats 30/30/31/31/29/31 rash, frame 65026 on trainerid 1227
frame 577205608, stats 30/31/29/31/30/31 rash, frame 151292 on trainerid 65228
frame 988536549, stats 31/29/31/31/31/30 rash, frame 5060 on trainerid 787
frame 1084427779, stats 30/31/31/31/31/30 rash, frame 16085 on trainerid 13270
frame 1116837636, stats 30/29/31/31/31/31 rash, frame 143102 on trainerid 19950
frame 1229191225, stats 31/30/30/31/31/31 rash, frame 56253 on trainerid 9436
frame 1585067498, stats 29/31/31/31/31/31 rash, frame 4217 on trainerid 52259
frame 2172861067, stats 31/31/31/31/31/29 rash, frame 19962 on trainerid 53955
frame 3497675233, stats 31/29/31/31/31/30 rash, frame 58807 on trainerid 10194
frame 3828666060, stats 31/31/31/31/28/31 rash, frame 12293 on trainerid 39049


rash all-but-1 31s:
frame 1585067498, stats 29/31/31/31/31/31 rash, frame 4217 on trainerid 52259
frame 421072740, stats 31/17/31/31/31/31 rash, frame 140870 on trainerid 6982
frame 1032709426, stats 31/31/23/31/31/31 rash, frame 21351 on trainerid 29949
frame 3828666060, stats 31/31/31/31/28/31 rash, frame 12293 on trainerid 39049
frame 2172861067, stats 31/31/31/31/31/29 rash, frame 19962 on trainerid 53955


modest (all 31s):
frame 2324046137, stats 31/31/31/31/31/31 modest, frame 15062 on trainerid 48245
frame 2964478064, stats 31/31/31/31/31/31 modest, frame 37295 on trainerid 21555
frame 3969456317, stats 31/31/31/31/31/31 modest, frame 70259 on trainerid 14578

mild:
frame 2499407975, stats 31/31/31/31/30/31 mild, frame 257887 on trainerid 1800
frame 2661541259, stats 31/31/31/31/30/30 mild, frame 28572 on trainerid 13393