I asked the same question before I brought my normal polishes. Everyone said that it is down to personal preferance, which it is, but I thught some opinions may help.
Anyway, I made a mistake when buying normal varnish. I went with a slightly cheaper brand, china glaze, and to be honest, I struggled to get on with it when applying it, I thought it was me, but I brought a nice red and light pink as well as an effect from CND, and now realise this is what I should have brought in the first place. I got on with it as soon as I started to apply the polish.
I now have around 15 china glaze polishes I need to use up or sell on cheap.
A lot of people said that they liked OPI, so I did also try an OPI polish, but I just didn't seem to get on with it. It was that it didn't go on well, I think it went on ok, I just preferred the CND polish once I had tried them. I will now be buying more CND polishes as and when I can afford them
I can't help with the gelish polishes as I use Shellac
Hope it helps with the normal polishes though x