waltz in 3/4 time, which quantize?

It might depend on whether it is a true waltz or not. Waltzes are typically in 6/8 time, since the three main beats do not fall on '1-3-5' (1-2-3 in 3/4). Instead, they fall on '1-2-5' (1, 1-and, and 3 in 3/4 time).

This little 'lilt' in the tempo was scandalous when waltzes first appeared, and bluebloods were outraged. It was the equivalent of Elvis shaking his hips in 1956.

So if it is a true waltz, I would Q to 8th notes, since the second 'beat' of a measure of waltz in 3/4 is actually the 8th note directly after the first beat. 1/4 notes would likely not work, moving your second beat away from '1-and' to '2'.

But Q to 8th notes also might not work. I would write the track in 6/8 and play the notes on 1-2-5 and Q that using 8th notes, which should work to get a true waltz meter working. Of course if your '2' beat falls closer to '3', then you will need to drag it back to '2', so that will not Q a true waltz meter from 'approximate' 3/4 (1-2-3).

If it is actually simple 3/4, then 1/4 note Q should work. You also might be able to find a swing value in 3/4 or 6/8 that will place beat '2' where it belongs for a true waltz format. So there may be a swing value that can take a track you have played in as straight 3/4 and change that to '1-2-5' or '1-1 and-3', making it conform to true waltz meter.

Let us know what worked for you.