opiates, most benzos, hallucinogens, pregabalin and a few stims (most notably MPH) all get more enjoyable for me with concurrent THC use.
i have a fairly sensitive stomach when it comes to opiates, so a bowl or two not only boost the euphoria, it also kills any nausea i might have. pregabalin on its own is rather boring in my opinion but combined with THC it's a shitload of fun, much more than the sum of its parts.
with benzos i've had mixed experiences. sometimes fairly boring benzo sedation was turned into something really euphoric while at other times an enjoyable benzo high was completely overpowered by the THC.
with stims it's mainly useful for the comedown, in the case of cocaine i never smoke while on it because for some reason it kills the euphoria. but as a drug for a stim crash it really is a second rate choice, i tend to need something a lot stronger and more chemical (read benzos or something like quetiapine) to effectively knock me out after stim use.