I would recommend GAFF or OPLS/A
Camps has outlined the OPLS FF, so I won't elaborate very much, but to say that when using LigParGen, a nice feature is that forcefields are made for many different MD programs such as openMM, Gromacs, CHARMM and others, so it is pretty easy. If you email Jorgensen, you can get a version of LigParGen that you can use on your own computer rather than having to use a web browser.
I also recommend the General Amber Force-Field (GAFF). It relies on using a program called Antechamber to make the intermolecular parameters from a lookup table, so geometry optimization does not matter - but, you then either make partial charges from restrained electrostatic potential (RESP) calculations, which does depend on the geometry, or AM1-BCC, which does not depend on the geometry you provide, but to a degree, depends on the conformer you provide. Part of AM1-BCC is that first it optimized geometry using AM1 which is semi-empirical and not all that accurate, but the BCC part is a correction, so they are usually okayish.
Antechamber is easyish to use.
I would avoid doing QM calculations for intermolecular parameters such as bonds, angles, torsions unless you really know what you are doing.
Here is a tutorial on GAFF and Ionic Liquids by David Case, a developer of AMBER
GAFF tutorial for Ionic Liquids
Here are some publications of Ionic Liquids with GAFF
The General AMBER Force Field GAFF can accurately calculate... ionic liquids
Interaction and Dynamics of ionic liquids...
I have also used GAFF for Ionic Liquids, but do not have a publication out yet, but will, one day, I keep telling myself... I can say that it works.