Fair-Rite 2643101902 mix 43 round core ideal for making EFHW transformers or common mode chokes.
 
This core is a good option for a 100w EFHW build. Offering better efficiency than a T140-43 core but at a higher weight of 67g.
 
An efficiency measurement from a Nano VNA is attached.
 
Also, a great core to use for a common mode choke using RG174.