Dunkin Donuts. Because Tim Horton's doesn't exist in the European Union (except for 3 branches in Spain - and I ain't going all the way to Madrid just for a coffee).
~~~~ Just another victim of the ambient morality ~~~~
I live in the UK and I've never heard of Tim Horton's and rarely see a Dunkin Donuts. I don't like coffee anyway so I couldn't say which was best as they'd both taste horrible to me.
I've also never heard of tim hortons. Never been to a dunkin donuts. Have tried Starbucks a few times and each time was terrible coffee - no wonder Starbucks closed down in my city.