Foster produced several fine saves and capped them off by blocking Steven Gerrard's second-half penalty before goals from Gareth McAuley and Romelu Lukaku secured victory for the Baggies.
The former England international has been in fine form in recent seasons for Birmingham and now Albion, but admits his performance against the Reds tops them all.
He said: "The penalty save probably just made it the best. I played a really good game for Blues against Chelsea at St Andrews 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 the final 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."
Foster's heroics helped West Brom end a run of eight matches without a win during a difficult time for the club which saw the Peter Odemwingie on-off transfer saga and Goran Popov's ban for spitting at Tottenham defender Kyle Walker.
Foster said: "It feels sweet, not only because of the difficult run but also because of the negativity that's been going on and has been played out in the media as well.
"We showed that we haven't let any of that affect us. We kept together as a team and a unit and we just carried on."