Generally speaking my favorite drugs are any solid, real deal psychedelic on it's own...but some combos with psychedelics were pretty badass, for example, LSD + DMT (after the L peak just ended), this is my favorite combination that doesn't involve a drug I won't do anymore, absolutely incredible, the L adds lucidity & ecstatic stimulation to the DMT, the DMT...well it blows your mind away, of course. Definitely a combo I want to do somewhat extensively when I'm able to get DMT again.
DOC + MXE (after DOC peak faded), my other favorite combo, although I don't do MXE anymore. Synergized perfectly, the DOC even balanced out MXE's destruction of coordination, whatever tasks I became involved in basically became my whole reality, plus the euphoria between these two is out of this world! Also produced a lot of what I think of as "classic psychedelic moments", reached that "holy" headspace on this combo basically every time I did it. 5-MeO-MiPT also went very well with MXE, in a more hedonistic, less classically psychedelic way (although it became very deep in the later half of the experience).
As far as the most out of this world, reality breaking hallucinogenic combo I've done, that definitely would have been DXM (708mg) + an unknown, very large quantity of 4-AcO-DMT, almost definitely over 50mg...that combo was the only time I've ever gotten fully out of my body, including dozens of times using high doses of DMT, MXE, DXM on it's own...I dosed the 4AD while already fried off the DXM, as it was coming up I thought I had gotten serotonin syndrome & was about to die, then when it peaked I was just gone, in the cosmos, so it's kind of like I did die, but I didn't care because I no longer had any concepts in my head of...anything, really. I remember turning into a lot of strange objects, among other things...it's really hard to remember well, happened like 2 years ago & I could barely remember it right after it happened as well. I know there's a lot of crazy shit I can't remember, though, when I came back for a few seconds I thought I'd blacked out, then I got hit with this energy I somehow associate with the waterfall, and suddenly could remember many scattered bits & pieces.
Outside of psychedelics, my favorite combination would definitely have to be amphetamines + (lots of) weed, they just synergize perfectly for me, balance each other out yet potentiate each other significantly as well. Perfect combination for just making music all day, or getting anything else done you need to get done, I suppose, but that's my favorite way to be productive.