I'd go for Downing I think. He took them for Middlesborough a few years back and did very well and as he's very much a confidence player getting a few goals would be good for him (conversely missing a pen would be a blow but I'd trust him).
If not probably big John or Gabby (which ever is playing), I always like the idea of strikers taking the pens, just by having to practice them it teaches them a bit of calmness under pressure which hopefully translates to 1-on-1s, etc.