End of January:
Cons:Shorter hours, ride refurbishments, too cool for swimming, late January was NOT as low-attended as years past in 2018 (or rather, Disney cut its staffing so that ride wait times were still rough)
Pros: Value pricing, Epcot Arts Festival
2nd Week of March:
Cons: Start of Spring break crowds, Higher Resort Prices
Pros: We went this exact week in 2018 and found crowds manageable, IF you Rope Drop; Beautiful weather, Epcot International Flower & Garden Festival, Longer park hours
End of May-Early June:
Cons: Hot weather (with more thunderstorms), Higher crowds than January (but better staffing, too)
Pros: Good for swimming, Mid-range resort prices, More expanded hours than January, but likely less than March
My pick? 2nd Week of March, if you're willing to Rope Drop. Because Easter is so late in 2019 (April 21), hours may not be quite as extended during the 2nd week of March next year, as they were for us.
We had a wonderful vacation this year, in spite of my concerns about crowds. Of course, we also really lucked out with weather!