IB - they even spread out your idle cash over multiple CD accounts to stay within the $200k FDIC limit... 1.6% or so
thoughts on series I savings bonds? https://www.treasurydirect.gov/indiv/products/prod_ibonds_glance.htm
Vanguard Prime money market has SEC yield of 2.09%. I bonds has annual limit of $10,000 per SSN, (additional 5k, if you are getting tax refund and use that refund to purchase IBond) which limits its use. Also you have to deal with TreasuryDirect which is pain