I found that a lot of people have their own taste for the texture of the primed surface; smooth (and somewhat reflective and gloss) surface, very rough, or fine rough (very matte).
I like very matte (enough tooth) surface which is better for applying oil paint on top of it.
I tried some hobby-specific primer on brush but not satisfied. most of them are designed for air-brush but allows brush on primer, like Vallejo's primer. They are too watery, not easy enough especially very smooth surface.
I heard old miniature painters used artist's gesso, so I bought Winsor&Newton's gesso but it seems to have some sand-like, very rough grain on it. It was useless unless you want to sand it after priming.
I tried Golden acrylic gesso (Golden was happily send some free material so I tried them initially), and it was good. It created enough matte/toothy surface for my taste.
I usually never dilute the primer. I applied Gesso like the way you brush your tooth with toothbrush, and very strongly rub the surface of the mini (I'm using completely ruined brush)