The former Manchester United player saved a Steven Gerrard penalty during the second half when the game was still goalless, before the Baggies netted twice during the closing 10 minutes for an impressive 2-0 win on Merseyside.
Gareth McAuley and Romelu Lukaku were on target for West Brom and Foster said: "That penalty save probably just made it the best.
"I played a really good game against Chelsea at St Andrew's a couple of years ago, which we won 1-0 when Lee Bowyer scored.
"I played well in that game and the Carling Cup final was great when Birmingham beat Arsenal in 2011.
"But with the penalty save and the fact we were playing at Anfield, that performance probably just tops it.
"There was also the fact we needed the win so badly which was probably just the icing on the cake. It was a really sweet one."