I am hesitant to voice my size preferences, because they have changed a few times over the years, and they may do so again as I learn and grow as a player. One thing that is becoming more apparent to me as I experience more ukuleles is that my preferences are in part not for specific sizes, but for specific instruments. The way they are built, the way they sound and feel.
That said, lately I have been preferring both sopranos and the bigger sizes (tenor/baritone/guitarlele, and possibly the guitarlele, but it only arrived today, too soon to say). My two sopranos are both in re-entrant tuning, one in C and one in D, and to me they are as different to my linear tuned tenor/baritone/guitarlele as a mandolin would be. There are distinct differences in how I play them, what I play on them, how they sound, and how I experience them. That is why I can't really say I prefer this or that size (of those I mentioned) to another, because they are completely different and hard to compare. If I could only play one instrument, it would at this moment be a soprano, but I'd miss the sustain and depth of my bigger instruments.