That is a tough choice...
I would probably pick Epcot with Magic Kingdom being a close second. I love the different countries and the variety of food choices. I love the ability sit back, have a glass of beer and watch the world go by but still be able to some great rides in Future World. When we vacation to WDW, we always spend an extra day in Epcot or hop back to Epcot for dinner.
The Magic Kingdom is great with the classic rides like Pirates of the Caribbean, Space Mountain, Haunted Mansion, and the Fantasyland rides. It is nice to know that if I cravea Dole Whip, there is always The Polynesian Village for that.
So...yes, I would pick Epcot.
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Prior: Offsite most every other years from 1972 - 2000
Most recent: Offsite 2001 & 2003 / ASM 2006 / Offsite 2009 / Poly 2011 / POR 2013 / CR 2014 / GF 2015 / Offsite 2017