For me (favourite beer in brackets)

Grey Trees in Aberdare (Afghan Pale Ale)
Mantle in Aberteifi (Dark Heart
Pipes in Cardiff (Californian Pale)
Purple Moose / Bragdy Mŵs Piws in Porthmadog (Dark Side Of The Moose)

Honourable mention to Wye Valley who, despite brewing the lush Butty Bach are actually just the wrong side of the border 🙂