ID11 is the normal beginners' recommendation. This is a metol-hydroquinone developer. Unfortunately, there seems no such developper exist in liquid form. Perhaps because of the short shell life.
You can go with one of the liquid phenidone developers, which are the "murderer" ones. Phenidone was detected as developing agent in the 40ies. Here we hve DDX from Ilford, or Ultrafin Plus from Tetenal or TMax developer from Kodak or LP Supergrain form Laborpartner or perhaps some more. Not each of these contains the pure phenidone some use a derivate of it, mostly pyrazolidone.
One of the powder equivalents of these developers is Microphen.