There's no straightforward answer to this question as everybody will have their own opinions. There is a lot to consider with sleeping bags such as seasons, climate, dampness, bag compression, materials, bag weight, your own tolerance to cold, ground insulation, shelter, your budget, touring, racing etc.
I can't say what to choose, but I can only say what I personally use. When I took part in the TCR, I used OMM Mountain Raid 1.0. I use the same bag for ultra running with weather conditions down to about 3c to 4c. I still use the same bag for touring as it compresses well and fits nicely inside my saddlebag under the seat. It isn't cheap, but with the money I've saved on accomodation it has paid for itself.
However, with hindsight for the TCR, I would have been fine with a bivy, Thermarest NeoAir XLite and a sleeping bag liner. The TCR is during the warmer months, and even up high in the mountains overnight I found the sleeping bag too hot. When it was extremely hot, it was cooler to cycle during the night and sleep during the day where I didn't even need a bivy bag, just a nice patch of grass in the shade.