Hi Calla Lily,
Of course most people are going to say their shard. And I do have to admit I play Baja (for 10 years, still there).
In all honesty though, it depends on what "type" of RP you are looking for. If you want traditional, in-character RP... I would say Pacific is likely your best bet, Napa Valley & Sonoma tying for second place, and then Baja coming in third. If you are into event RP, then flip that around... Baja would be the best bet for events, and the others tie equally.
They all have room for housing. Pacific and Baja are both average medium populated shards, Napa and Sonoma only slightly behind. Any of them can flip-flop on any given day.
PvM? Baja. For the reason that Baja of course has a lot of US players (being a US shard), but it also pings really well to Asia... and for a US shard has a lot of English-speaking Asians that also play there. The diversity of players and playstyles makes Baja the place to learn strategies & tactics especially for PvM... and even PvP if you want to get into it.
Any of the four would be a great shard to play on, there are great players on all of them. Make a character on all of them... and give each a try.