Merino, Merino, Merino and without any hesitation - Icebreaker is my 'go to'.
The base layer is the wicking layer, it moves moisture away from your skin which keeps you warm in the cold and cool in the heat.
Most importantly for me, as I spend so much time around and in the water, Merino dries very quickly and is not 'cold' when wet like cotton.
Why Icebreaker, possibly habit..... Yes I've tried a few others but I always seem to come back to my Icebreaker tops some of which are approaching 5 years old and have lasted perfectly.
I go for the 200 weight: as a single (crew or zip) they're great as a single layer on warmer days and double up (crew neck below zip neck) when it get chillier.
Available in Ladies and Gents fits and a large range of colours and designs.
(a word of advice: avoid the deep red / burgandy if you live in Colchester as our friendly Para's might give you an odd look or two if you're carry extra pounds as I am!)