I voted for phthalo blue, the one that I cannot do without. In other words, if I had push to pick ONE blue only, I would choose PB15.
For blue I always used Lefranc or Blockx, due to poppy oil as a binder (I’m afraid the yellowing about blue, violet and white), but recently I’m enjoying Old Holland Sheveningen blue deep, a wonderful color ground in linseed oil; I think that with a single 40ml. tube I’ll paint a few hundred paintings!

It’s different whether it’s oil or watercolor.

In oil paint it’s easier to casually mix colors and fine tune the mixes with controlled adjustments. I typically just use ultramarine in oil paint even though I have plenty of other options. It’s inexpensive and gets the job done well. It may not be ideal for mixing greens, but then it’s easy enough to add a green to the palette and use that as a base for green mixes.

In watercolor it’s different because it’s harder to be exact with mixes and you have to consider granulation as an aspect of the paint. I have a preference for cerulean because depending on the variety it’s a great sky color and fairly light in masstone, so it can be applied more liberally without accidentally going too dark. Indigo, especially PB66 (the synthetic version of natural indigo), is another blue that I use a lot in watercolor and is good with a low chroma palette of yellow and red ochres or a burnt umber, which is just the kind of palette I prefer. Then again, ultramarine is an inexpensive workhorse without the concerns of cobalt in the waste water during disposal and is more widely useful than indigo. Plus, adding a little green earth, which is weak tinting, into ultramarine and diluting it well gets a very similar color and granulation to cerulean while being much more transparent. I almost want to say cerulean for the color straight from the tube, or indigo because I use it so much, but if by favorite you mean a blue I’d want as my only option then I’d probably have to go with ultramarine.

When I was taking art classes a long time ago I remember switching from ultramarine to Prussian blue because it seemed different and exotic, and then later to indanthrone blue for the same reason. Eventually I tried pretty much every blue pigment made. Somehow I seem to have come back around to choosing ultramarine.

If I had to choose just one blue that I had to live/work with the rest of my life…possibly Ultramarine Blue. But since the question is which is your favorite, it’s probably Indanthrone / Indanthrene / Anthraquinone Blue PB60 for its combination of novelty and its usefulness.

It’s moderately chromatic, so it doesn’t need to be constantly dulled down – so it’s natural and easy to use (especially in tonalism), with a huge value range. Yet applied thickly undiluted or untinted, it’s an incredibly deep & rich blue…deeper than Ultramarine.

cerulean
so nice straight from the jar

Golden Paints Anthraquinone Blue without a doubt. It is such a beautiful, velvety deep blue with the widest range of gorgeous tones. I love it

Cobalt blue. Semi-opaque, pretty but not too chromatic, and unassuming in mixtures with earth colors. Makes wonderful dull cyans mixed with terre verte or cobalt green. Smashing for portraiture.

I voted for Ultramarine Blue (which has more tendencies toward the primary color, Cyan, than toward actual Blue). While not being a true primary, I find Ultramarine Blue to be one of the most useful “Blues”.
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In paint, that (CMY primaries) would be Cyan: Thalo Blue (PB15). Magenta: Permanent Rose 502, or Thalo Red Rose (each of which are PV19). Yellow: W & N Transparent Yellow 653 (PY128).
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I’ve seen your paintings here, so I know what you mean. I’m still doing many color mixing exercises. Lately, I’ve been thinking that if I had to toss a color, it would be ultramarine blue. But it doesn’t seem to lean green.

As you noted, UB does well making neutrals with some earths, and makes some nice greens with yellow. But I don’t like UB’s own color, especially when lightened. However hard it is to use Phthalo Blue GS (PB15:3 or PB15:4, rather than PB15:1), I find that with careful usage, I get colors I like better. Its green mixtures are easily controlled with Quinacridone Violet (PV19) which I prefer to the red or rose shades, since I don’t need a lot of highly chromatic reds or oranges, or purples.

Instead of mixing neutrals across an UB-Sienna/Umber axis, I’m trying Phthalo Blue GS against Venetian Red these days.