Lightest 150 merino base layer top.

Maybe check out one of the merino blends like the Rab Meco line. The merino/cocona mix is lighter than pure merino, and retains pretty much all of the benefits of wool, with the added benefit of faster dry times. The 120g/m2 weight stuff is comparable to 150g/m2 weight merino, and the Meco 120 long sleeve weighs 137g for a men's medium.

That said, if you must have pure merino, I love my Icebreaker Hopper Lite t-shirt (150g/m2) – the mens small I have weighs 132.2g and the relaxed fit means it fits more like a medium. The LS Altas is probably the most comparable long-sleeve design from icebreaker (I'm a big fan of raglan sleeves) – although the Altas line is more of a next to skin fit, so sizing up would be appropriate (I'd need a large in the Altas to get for a similar fit to my size small Hopper Lite). My guess would be 170-185g for the Altas in a comparable fit to my hopper lite.

The only real weakness to 150g/m2 merino (and pretty much all lightweight baselayer fabrics) is durability – expect pilling for high wear areas like under pack straps, etc. I usually find myself reaching for 180-200g/m2 tops for that reason (I'm actually a pretty big fan of my Stoic Merino Crew for backcountry use). They really don't weight that much more, but have held up much better to use.

Thank you for your suggestions. I will look more closely at your suggestions this evening. I think I am looking for a closer to skin fit. I actually own a 1/4 zip 200g icebreaker bodyfit that has the fit I am looking for but I was hoping to shed a few ounces by doing a non zip crew.

It sounds like you feel that the weight penalty of the 200g is worth it for durability purposes. Durability is a concern, this will be used for a thru hike.

Here are my weights:

Weights in grams/ounces:

Arcterx Zip XL chestpkt _322 11.4
Stoic 150 Zip XL _238 8.4
I/O Merino zip Hoody XL _228 8.0 tighter weave, stretch
Icebr 150 crew Xl _200 7.1
Ibex zip XL _188 6.6 looser weave
Meco 120 crew _172 6.1
SW grey Shortslv _140 4.9
Pata merino1 crew ls XL _128 4.5 70% wool? HUGE!

Ibex Shortslv bike jersey w M 204 7.2
Icebr 200 zip ls Wms M 187 6.6
SW microwt? zip ls wms M 146 5.1

you'll get a little more longevity (and performance) w/ a wool/syn blend, the Patagonia Merino 1 (blend) is a pretty light choice

I picked up a Merino 1 and my girlfriend went with a Meco 120. Both are great. Thanks a lot.