No contest, Senna would've crushed everyone and definitely would've won the 94/95/96 drivers' titles with Willams. He'd have maybe stayed on at Williams in 1997, likely winning that season too.
Since Williams were on poor form in 1998, Senna would've gone back to Mclaren for 1998, as there's no way he'd have stayed at Williams after they lost the Renault engines. Senna would've easily adapted to the grooved tyres that came in for the 98 season, and as McLaren were so dominant that year he probably be winning another title.
He always said he wanted to finish his career at Ferrari, so he'd have moved there for 1999 and 2000, where I think he'd have won races but not another title. He'd have been in his late 30s by that stage, therefore would've retired as a 6, 7 or 8 times champion with over 100 wins,150+ poles and 10+ Monaco victories.