The large spool of grey thread is one of the best investments. I use that to sew up seams. Grey is a rather neutral tone and can sew darks to lights, darks to darks, lights to lights. It blends so well with everything you sew.
I have also used Sulky blendables for machine quilting. They are very nice to work with, but a bit expensive for everyday quilting. Those I keep mostly for special projects. I am starting to collect them. I treat myself to one or two everytime I go to Sewing Machines, etc., in Burlington.
Do I use invisible? Rarely. Someone once commented that they are leary of using invisible threads as they don't know how well it will perform in years to come. Will it rip the fabric? That was in the days when invisible thread was like fishing line. Now it is finer and a lot more pliable. When I do use invisible I buy Sulky. I have both colours, clear and smoke. Clear on lights, smoke on darks. I use a bobbin thread when I sew with invisibles......once again made by Sulky.
As with everything else, thread is a personal choice. You may find that you love Mettler, Aurifil, Coats and Clark. Do remember though, it isn't worth buying that cheap 99 cent spool of thread. If you pay $15-$18 a metre for fabric, why ever would you use cheap thread to sew it together.
Make sure your thread is fresh when you buy it. Test it. Pull on a strand. Old thread and even new thread that is of poor quality will break easily. The fibres have broken down or were just never there in the first place.