My original equipment seatpost is described as the following: Suspension, 45mm travel, 350mm x 27.2mm, alloy clamp with QR seatpin and is pictured below. I need to get rid of this movable menace and am looking for a good, relatively lightweight replacement.

Will any 350mm x 27.2mm seatpost work as a replacement? I'm looking for a good carbon seatpost under $100 or the best/lightest aluminum seatpost.

Yes, any 27.2mm, just pick the length you want, Thomson are generally considered the best Aluminum seatpost around, for carbon there is plenty of choice, although your budget may be a little low for a branded one.

What about the Ritchey Pro Carbon? Seems not to be well received.

I assume a Thompson Elite 27.2 x 330 would work?

https://www.nashbar.com/bikes/Product...2#ReviewHeader

Carbon is ok but has been known to seize in AL bikes. It could also have slippage issues.
There are plenty of opinions on both sides of this issue.

If Thomson is not your thing check out Bold Precision. He makes custom TI posts. Pricey, but well received. I emailed him a few questions and he responded promptly. All his stuff is made in America one at a time by him.

There is also KCNC and WOOdman. WOOdman makes a good $50 post. KCNC makes really light TI posts. Not lighter than Bold though.

Thanks, CACycling! I ordered one based on the price and glowing reviews.