I'd say Frankfurt because on today's tracks you're going north of a great circle route but first I'd ask:
1. What sort of engine problem?
2. How many engines do I have? (If I'm BA I'd just keep going to JFK.)
3. How much dump time do I need?
4. What's the weather at ATH?
5. What's the weather in FRA?
6. What's the weather in AMS? If you're going to be dumping for 30 minutes, you may as well be going somewhere.