Years ago I switched from Citadel to Vallejo's Game Color line and initially I was really happy with the change. After a while, however, I needed to buy a few more paints and... not sure what happened exactly. I don't know if they changed the formula, or if some pots had just been sitting idle for too long in the store (as Talys suggested), but I started getting way more watered paints. With some of them it was flat out ridiculous, to the point that it's been like five years (at least) and I'm still using Dark Green only for washes
About a year and a half ago I switched to Model Color and overall I've been very happy again, to the point that right now when I need to replace a spent Game Color pot I just pick the equivalent (or closest) Model Color one.
Initially I missed
GW's metals, as I've found Vallejo's silver metals to be terrible (the copper-bronze metals are okay-ish). However it's been a while since I stopped using metallic colors for my models, nowadays I just prefer scales of greys for silver metals and scales of reddish browns for copper-bronze. I still own a few Vallejo metals which I use for small details when I don't want to be bothered to mix.