Polystralia is good but they're only "awesome" on the right map. So if you play Atlantean maps (most if not all capitals will end up on the coast) with the continent surveyor and manage to pop the reverse polarity quest reward quickly and don't get involved in early war with anybody that has gunboats yeah, they're pretty awesome. And, it's pretty easy to get that setup.
Switch maps and you may find yourself with no early external routes, two extra internal routes is still very good but it's no longer an early bonus then since you need four cities. It's also backwards if you're stuck with only internal route since the ideal setup there is to have the new city routes to the capital for the "super city" effect. And, once you have four additional cities the value of those two extra trade routes is considerably diminished. You also have to consider the science and energy provided to the trade partner. If your external route is coming to me I am also getting a boost in science and energy and although it may not be much it is a lot more than I will get if your spy is stealing from me.
ARC has the advantage of the simple fact that you can always see at least one city belonging to each player until one of them loses their capital. So, regardless of the map type you have somewhere to send your spies. Spies are immune to aliens, gunboats, planet carvers and... well, everything except an enemy spy if there is one in the target city.
In the end though, it doesn't really matter.
Any sponsor is capable of achieving any victory condition and the advantages of any one of them are really small enough that you're only talking about shaving a handful of turns off your victory in most situations. Sure, you can come up with the perfect conditions to make any one of them really shine but as it stands now you're not really an worse off just picking the sponsor that has the color that you like most.
It isn't hard to get more early trade routes than the Polystralian player. It may seem unlikely or irrelevant but it's no more unlikely than having a game that plays perfectly into your sponsor and chosen strategy. In my last game as the Slavs I popped a solar collector on turn 2 so I took a gamble and used it on pioneering (forgive me) and it actually turned into one of the most amazing early expansion games I've ever had with two cities and two outposts up at turn 50 at standard speed. Resource pods were the real reason though- that little random element can have a much bigger impact on the early game than your sponsor choice (or alcohol induced tech choices).
Edit: Oh, and I picked FI as the best. Virtues are very good and everybody can benefit from getting a lot of them so getting a couple extra free techs along the way is a nice, reliable bonus.