I agree with Amoeba about 2 being the prime game in the series. Though honestly, none of the Star Oceans had very deep story lines. The 10 Wise men in SO2 are highly undeveloped characters, and yet they're the main villains in the game, that is pretty much a staple for all the games.
SO3 was just... well boring. Not to mention none of the characters are likable at all. (Abel was probably the coolest one there.) They tried to make the creation system make.. more realistic I guess? Well it fell on its face because it was no longer fun.
SO4 is just as boring if not more so. Again, zero likable characters I got 20 or so hours in and the plot was still stupid. Edge was crying about how he sucks. Blah blah blah. It's pretty lame. Oh and the graphical style is just odd. The characters look so weird and stiff. It's almost painful to watch. I remember getting lost in a dungeon because EVERYTHING looks the same. EVERYTHING. They tried so hard to make it pretty (I hope?) and I can't even see the switches to open anything! AHHHHHHHH FRUSTRATING.
I think the series could use a serious reboot. If they're going to use a anime style art and character designs... for the love of god make it look as such. Don't try to make them realistic Tri-Ace. FOR THE LOVE OF GOD.
Great rpg game mechanics is a tri-ace specialty. Story telling... not so much. Though they do surprise you occasionally. Case in point, Valkyrie Profile: Covenant of the Plume.
Haven't played that one. But VP: Lenneth is one of my all time favorite games... like ever. Silmeria isn't quite as good but it's not bad.
I think my two favorite RPG series would have to be Tales and Suikoden (COME ON SUIKODEN 6!)