If we’re being serious then canonically it should be Patty from third grade, plain and simple.
However, if we’re following based on production timeline then it goes like this before every retcon in each series:
OS: Kai Green - Kai was Ben’s first and only known crush before marrying her in the future as Ben 10,000.
UAF: Elena Validus - Elena was Ben’s first crush, then sometime later after she moved out, Ben went on the summer trip when he was 10 and met Kai on the way as another crush and by UAF….. you know how the rest works out….
OV: Patty from 3rd Grade - She’s Ben’s first crush.