This depends on the situation, but generally after smart:chain.
Due to the technical limitations of DAWs, a plugin can only ever “hear” an incoming audio signal, with no regard to whatever effects are applied afterwards. smart:chain strives to create a clean and balanced mix as a starting point. An effects plugin will always have a (wanted) effect on the frequency makeup or dynamics of a track, which smart:chain will then try to “fix”.
Do keep in mind that effects plugins will usually also affect the level of a track, which smart:chain’s Auto Leveling will not be able to compensate for if the effects is added after smart:chain.