-Highloot definitely helps cut down the grind some.
-Socketables will undermine some of the balance, but gives more freedom for how you gear.
-Pet and Fishing Overhaul can mess with balance a bit if you abuse the consumable fish buffs, but otherwise brings fun and interesting stuff for your pet.
-100x XP and 10x Fame is great if you just want to skip to endgame content, but it may hurt your learning curve if you aren't already comfortable with each class' abilities.
-General Passive v.9 (the Synergies compatible one) will also mess with balance a fair bit, but it's great for builds that have a lot of left over points that you don't want to waste on skills/passives you won't use or care about.
-Free Entrance Resurrection is probably one of the few that I wouldn't play without. There's enough stuff to spend time grinding on that I just don't see a reason for gold to be one of those issues. Doubly true since most places in the game would only have you go to town and port back to save the gold anyway, so what you really save is time in loading screens.
-LurkerHUD is probably the other major one that I would rather not play without. It's simple, unobtrusive, and helpful. It also negates the need for a few other mods as they become redundant.
In general anything that lets you do more, or that isn't specifically designed with Synergies in mind, will likely upset the game balance and possibly introduce bugs/mod conflicts. Custom classes are especially guilty of upsetting balance since the people that make them are generally very amateur at game design, and don't have the full context or experience of Synergies to properly balance against it.