what is the go to thermal leggings of choice?

I like power stretch tights, like MHW (their proprietary fabric rather than actual Power Stretch but same thing):

http://www.mountainhardwear.com/mens-stretch-thermal-tight-OM5164.html

If not those, I think I'd take a cycling tight with a brushed inner.

I've been very happy with under armour tights – durable and warm on the trail and the bike

I'm still a Merino Wool fanatic. I use Ibex's Woolies layer, the ribbed fit is very warm and they seem to weigh less than the Smartwool alternatives, but I haven't got a scale.

That said, there's a super budget item that works great for me as well:

Uniqlo HeatTech Long Johns:

http://www.uniqlo.com/uk/store/goods/078003

I'm a big fan of the golite fleece lined tights… on clearance right now.

I use Terramar silk long johns which are probably the lightest option out there at 2-3oz. Obviously they won't be as warm as merino wool, but they're great for sleeping and doing some light hiking. At only 2oz, I find that I take them more times than not.

http://tinyurl.com/qykpahf

Ryan

There are lots of choices, from silkweight polyester like Capilene 1 and on up to Capilene 4 and R1, or even 200w fleece if you want to get really crazy— all progressively heavier or course. For really cold weather and sleep, I have a pair of the R1/Dry Power military long johns. REI and Cabela offer Dry Power long johns in lighter weights than the R1. Capilene 2 would be a good middle of the road choice for 3 season use.

You could over-analyze the lighter stuff. For less than frigid weather, any good wicking material will add another layer of warm air next to your skin. I would just go for light cheap and wicking if it's not below freezing. I assume you are in Australia and I wouldn't go for some expensive import unless you really want to split hairs on weight.

I like the light stuff under rain pants rather than wearing typical Supplex nylon hiking pants. That reduces bulk and makes a slippery layer, while keeping the cold fabric off your legs and wicking way moisture.

These are probably the next lightest thing that doesn't look like pantyhose like the silk ones (I have both). They are pretty warm for what the are and would work as regular tights (under shorts) or inside your bag. Size large is 3.4 oz. I also love the Patagonia SW ones, but they are more weight. They are very durable though.

For more warmth obviously you need to go heavier.

When combined with wind pants or rain pants, Cap 4 or thicker fishnet type pants (think Wiggy or Bryjne) are surprisingly warm, and have unbeatable moisture management for intense hiking at decent weights (well, Wiggy's are on the heavier side).

Alternatives are any decent, thinner merino-higher synthetic ratio blends. Advice applies for tops too.

I wear Power Stretch tights all winter. Awesome for around the house, around camp, or a a super warm baselayer.

I am blown away by how much of a range the new Capilene 4 gridded PowerDry fabric is. Lighter than Power Stretch (and long-john looking), the stuff is amazing for temps from 60 degrees as PJs to 10 degrees (as a base layer). The fluffy grids provide warmth, the channels between the grids let it breath. That's what I would take as a first option for lightweight warmth.

Seems like there is no real 1 that stands out more like a personal preference. Will check them out, ta for the comments :)

Well, 2 folks recommended Capilene 4 or similar fabric (light, breathable Polartec Power Dry High Efficiency [grid fleece with larger voids]). MEC and some others also make some stuff with it. Considering how versatile the stuff is, how warm or cool it can be, might be something to look into. Get the stuff with polygiene permanent treatment and you have pretty decent and fairly durable odor control too.

And, the Polartech PowerDry High Efficiency and the Polartech PowerStretch are very close cousins Basically, the PowerDry is a lighter version of PowerStretch with some of the fleece shaved away to create a grid pattern. The R1 gridded fleece is PowerStretch with some of the fleece shaved away.

I wear the lightest merino wool long john bottoms most of the time, but in very cold weather I love my Power Stretch tights.