Gwyn was very difficult to pill, and some of the flavors of the medications couldn't really be disguised too much by the chewy treats, so those often weren't an option.
We did liquids successfully for a *lot* of years; Gwyn mostly liked the chicken flavor, which was odd because she preferred tuna as a treat. One really nice thing with the liquids is that it's easy to adjust dosages accurately, so if you want to give 10% more or 10% less, you just adjust the syringe accordingly. We cut back on the oral liquids when Gwyn went up to something like 10 syringes a day; it was just too hard to keep up with everything, so we switched a bunch of things to more readily available pills or powders.
We used transdermal gels when inappetance was an issue; we didn't want to force things into Gwyn's mouth and potentially make her even more stubborn about eating stuff, which had happened several times previously. I think the things we mostly used the transdermals for were appetite stimulants and steroids, but there were a couple of things as well, I just don't remember what they were.