Looking for an FX plugin that can modulate the signal like in the example below (the sound on the very first synth.

 

It doesn't sound like just one effect type. It sounds a little underwater, filtered, phasing, and maybe even a little lo-fi reverb'd.

If I wanted one plugin to make that sound, I would use SugarBytes Wow2. If you aren't a little conservative in how you use it, it makes almost everything sound like that.

 

My guess is the output bus (being fed by what sounds like 2 layers) has an effect chain going
Filter FX {Metafilter/Filterfreak 2/Volcano/Etc.} -----> Plate Reverb or Delay ---> Compressor --> Phaser

*possibly a freq shifter somewhere in the equation*

It's not as simple as dropping in one plugin, that sound is likely the sum of either several layers going through stacks of effects with a minimal-moderate wetness %, send signals to potentially various busses with seperate fx layers from one another, and most importantly AUTOMATION which reigns supreme in achieving more compelling outcomes with the specific sounds you're using.

The chain I guess-tamated above might help you achieve what I think ur looking for if automated carefully. Experiment with automating Dry/Wet knobs for different fx plus in an out of one another. I find keeping the wet percentge below 50% in my stacks allows me to control the direction/vision for the final sound.

{Might help to look at the eq response of the song as well}