Are CME BTC futures the best way for US traders to trade/hold BTC with leverage?

Which brokers support it and at what margin?

Amp does not, CME exchange requirement is 28k Maintenance. TD wants 45k, IB is 50k or so.

Any other options that work to trade this longer term with lower spread/roll costs?

Futures Operator 

Are CME BTC futures the best way for US traders to trade/hold BTC with leverage? Which brokers support it and at what margin? Amp does not, CME exchange requirement is 28k Maintenance. TD wants 45k, IB is 50k or so. Any other options that work to trade this longer term with lower spread/roll costs?

You'll get the lowest margin requirements at smaller crypto exchanges like BitMex / Kraken Futures. Both have built the reputation of one the most reliable crypto exchanges.
Of course both are nowhere near the level of security you get with a FCM and a segregated bank account.

You can check out the BitMEX contract specs here: https://www.bitmex.com/app/contract/XBTUSD
The contract size is $1, and the minimum deposit is a few hundred, so the initial investment is much smaller. Kraken Futures has similar terms. Data is free.

Besides the initial investment & fund security you should also consider the differences in market structure between the CME & these crypto exchanges. Last time I checked (~year ago) BitMEX & Kraken actually pay you when your limit orders get executed - they pay a small fee for all liquidity providers. Market orders (liquidity takers) come with a big comission which last time I checked was equivalent to 4-5 ticks for the XBT contract.

All of this info is a bit old, so please verify it before making a decision. If you want to invest a significant chunk of money go with the CME. If you just want to try bitcoin futures out go for BitMEX / Kraken.

Edit: now I remembered that BitMEX doesn't accept US clients