The best pint of Guinness has to be where it's made, in the bar at the top of the Guinness Storehouse museum. The view helps.

Beyond that, Guinness is nitrokeg these days anyway, so there really isn't much variation- assuming the pub does the basics properly (proper refrigeration, clean the pipes properly, etc). I can't tell the difference.

I prefer Kilkenny Red anyway.

For pubs, the Temple Bar area is alright (I like the music in Gogarty's and I like the ale in the Porterhouse) but you're in the tourist trap and pay tourist trap prices.