This is very subjective and there’s no single answer. If you want an aggressive, more accentuated display curve, then get an 1800R or a 1500R, even a 1000R if you can find one. That kind of curvature is more likely to give you the much talked about “immersion factor”, especially in gaming.
As for 3800R and ultrawide, those are awesome for a combination of massively wide screen real estate and a slight curve that better accommodates natural human viewing.
In summation, if you want a curved, non-ultrawide display then 1800R is ideal. If you do want ultrawide, then 3800R is by far the most popular choice, and for good reason – it’s a comfortable and impressive middle ground between size and curvature.