Gildan Softstyle are my current faves. About £4 from amazon posted, not thick though, very soft and thin but very comfy.
However as you want heavyweight, Gildan Premium heavyweight or heavy cotton are very thick for a T, I have tons of “ultra cotton” which are tad thinner and comfy all year round.
I’ve switched form Fruit of the Loom to Gildan as they are better quality and stitch. Generally better quality stuff has no stitch on the side from the armpits down, and flat covered seams around the neck (it’s the last bit that persuaded me to Gildan from FOTL!)
I work in IT and I wear a lot of plain t shirts to work btw.