Been planting brassicas in mixes for the past 5 years. My favorite for my area, by far, is T-raptor. It's a hybrid turnip/rape, which produces very little bulb and lots of leaf. One year i grazed it 4 times. Probably count on 2 really good ones, if you don't destroy it first round. Like said earlier, lots of moisture in the brassicas. You must strip or time graze it and balance with palatable hay/dry matter. Spring oats (i've used Everleaf) mixed with T-raptor is a homerun, IMO, but you have to plant it on time. If you plant too late, you will be disappointed.
Grazing radish, or tillage radish, is the most aggressive growing brassica i've planted, but they the cattle will choose just about everything else green over it. It will produce crazy biomass with very little inputs/fertility.
I also liked Impact Forage Collards and Purple top turnips. The latter doesn't regrow very well.
As also mentioned before, if you plant brassicas during the right time of year, they will jump up and outcompete most everything else. (the spring oat kept up pretty well) You can plant ryegrass underneath, and it will come on later in the year when the brassica has been grazed off.