A lot depends on where you are and what land and facilities you have available. Lleyns are pretty hardy, but are prone to multiple births so you don't want to be on poor ground when they lamb; Charollais are not the hardiest and can suffer from sunburn on exposed flesh - but in a temperate area they are just grand; these days pedigree Texels are moving towards sponging, AI, huge lambs so increasingly needing caesarians...
Where are you, what land and facilities do you have, and what are your objectives? (Making an income / having some fun / showing / rearing lamb for the freezer - what?)