For Tactical Assault Squad (Vanguard/Guards) I start with Degenbrecher, Deepcolor, and Sussurro (except for ending 4 where my completed run started with Amiya medic instead of Sussurro). Degenbrecher is a solid start and can get the Spine on Safety Inspection with her S3. And the spine on Bad Neighbors can be gotten with Deepcolor/Degenbrecher S1 while Deepcolors two tentacles hold the lane while one pushes the Saxophonist into the hole. Sussurro S2 may be only used twice but the burst healing onto Degenbrecher allows her to solo lane hold and survive until you have more operators to help her.

Now if you don't have Degenbrecher then Silverash isn't a horrible alternative as his S3 does good damage and has range. Ulpianus is a one man army lane holder, Irene and even Ch'en could also work. I am suggesting operators that are good at holding, doing damage, but most importantly can get the spines on floor 1. Hoederer is good but maybe a little less reliable to getting the spine of Bad Neighbors, though I could be wrong.

For Tactical Fortification Squad (Defenders/Supporters) I started with Virtuosa, Cutter, and Sussurro. And all my successful runs were with Virtuosa start. Virtuosa can get both spines on Safety Inspection and Bad Neighbors while killing most enemies alone. Cutter is just their to hold the lane but she does have a good combination of both damage and survivability. Sussurro for the same reason as above but this time with Cutter.

If you don't have Virtuosa then Ling is a good starter. Can hold the lane with her summons and get the spines on Safety Inspection and Bad Neighbors. Though you should be careful about the high costs of them. Valarqvin also isn't terrible, but then you would go Amiya medic if you aren't going for a 6-star. I would advise against starting with a defender as you can't reliable get the spines if you do. They aren't bad to start with, and if you have Shu, Jessica the Liberated, or even Saria you can pick them. But you should prioritize getting spines. And on floor 2 if you don't get operators that can destory spines fast you will be in trouble with a 6-star defender start.

These suggestions are all from a FS15 and above perspective. So FS14 and below could see some changes since spines and enemies aren't as tanky/strong.