A few rows back from the front in the stalls so you don't get a sore neck from constantly looking up, close to centre as possible so you can see the actors/performers - what are you going to see? Or the front row of the dress circle if you don't get squiffy with heights.
UNLESS it's for a panto, in which case RIGHT at the front is my favourite for heckling! I got over excited last year when all the kids were shouting "She did it!" at one of the Dames, the noise died down and I shouted "She's got an Adam's apple!". He dragged me up when they were doing the kid's things but I got some free sweets so I was happy. The missus however, was mortified.
I LOVE PANTO!