There isn't really anything better than Merino wool as a base layer. Warm, highly breathable, doesn't smell etc. Icebreaker are my favourite, but there are often some bargains to be found in Aldi or Mountain Warehouse, although I don't find their fit to be as consistent as the top brands. Most outdoor clothing brands are doing Merino or Merino blend tops these days so you're best trying stuff on to see what fits you best. I like light weight Merino T shirts in the summer too, so getting a few different weights and layering in the winter can be quite useful.