Cranks

We have two brands - Clipper, SOMA's brand, and Silver, our brand, which is what we use on 90% of the bikes we build up.  The Silvers are available in both a 110/74 and 104/64 bolt circle diameter (BCD). The 110s are the most popular but if you want super low gears, go for the 104/64 2 piecer; the 64 BCD inner allows for a 22t small ring.

The Clippers don't have as nice of a finish and have a hidden fifth bolt that's annoying when it comes time to change chainrings, but they're a great value.

All cranks listed are compatible up to 10 speeds.

If all this is confusing to you, just get the Silver 38/24 wide/low double. That one has a good enough range for almost everybody and you'll get a handy chainring guard too.

Here's a gear inch calculator chart.