Quite simply here are the easiest cases to watercool without modding:

1) HAF932/ATCS840/Cosmos Series, straight up triple 120 rad at the top - good to go
2) TJ07 - triple or quad rad at the bottom, not cutting necessary - with some handy work it can easily fit another dual fan rad on top of the triple/quad on the non PSU side.
3) Lian-Li P80 - triple or quad at the front, loads of space, easy as pie. - expandable with modding to 2 large radiators.