Probably a OHP class Frigate - they are 30 years old at least now most of them. Great ships in their day, but needing replacement now, and not with just the LCS class IMO. The current Frigs in the US Navy have had their SM1 missile system removed, so they have no missile capability, just the torpedo mounts, the helo, the CIWS, and the dinky little gun mounted in the superstructure in the middle of the ship.
A new Frigate would be great, especially with this rail gun system from the OP in the thread. A new class of Frigates in the US Navy with the newer AMDR (air missile defense radar) systems out there, with the new SM2/3/6/etc class missiles, in a small VLS system that can have a wide variety of weapons, would be fantastic. Anti sub missiles like the modern Asrocs, the close range ESSM Sea Sparrow missiles in quad pacs of 4 in each VLS launch tube, SM2/3/6 long range air defense shots, and even Harpoon and other upcoming missiles like the new Tomahawk anti ship missile replacement that's coming. A cell of about 40 of these, along with a rail gun, new laser weapon system, a good 76mm gun, along with a few 25mm mounts, can easily be put on a modern 4000 to 7000 ton class Frigate. The hard part is generating the electric power to run the AMDR, Rail gun, and laser, but they are working on this tech rapidly enough that they could put these systems on a modern Frigate. I hope the US Navy will be able to build something along these lines soon.
Something like this may be slower than the LCS, but it'll have a much stronger and more survivable hull, have far more capability and firepower, and more sailors to do the work, as the recent first deployment of the LCS showed that even with the crew increase of 20% the Navy recently did, it still isn't enough, and imagine if there was a damage control drill where every man counts - not good. So, a blue water proper Frigate is still needed, as the LCS just can't do that mission, and doesn't have the firepower or survivability to accomplish much. For what it is, a clse to shore small ship threat fighting machine, it'll suffice, being so fast, and once they get the guns working better. But out to sea in blue water, I think it's been established that the LCS just won't cut it vs modern warships and subs, outside of its littoral environment.