AT
Lynx

RME, MOTU, and Lynx are all excellent performers.  All offer low round-trip latency.
 
If you just need stereo analog I/O, Lynx E22 would be a good choice (goes down to a 32-sample ASIO buffer size).
 

Well, for me, coming in at about $1000 cheaper than the RME, is the Mackie Onyx Blackbird. Crystal clear sound with virtually non-existent latency, in a firewire-400 platform. Lots of great options for routing and daisy-chaining.
 
I saw someone already brought up wi-fi. That is definitely a latency monster.
 
Of course it should go without saying, but it's always crucial to trim the digital "fat" from your PC's background and redundancy functions. I always disconnect the internet entirely and disable virus software as well. I've eliminated as many useless programs and applications as possible. In all, I find that I never run into the least noticeable bit of latency until I start piling on tracks with soft synths and adding a lot of effects plug-ins.
 
Another huge help is to freeze tracks that are complete, or at least not the focus of the day's work.
 
I still have an old interface from 2004 that has only 2 audio inputs. One of the first models on the market to include 24bit/96k conversion. It works flawlessly with my PC set up. Like it was just released on the market.
 
Moral of the story ... dig around in the software and of course maintain drivers, and you'll solve most of your latency problems before you even consider an interface.

If you have working Thunderbolt your can't go wrong with Focusrite Clarret.

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Thunderbolt provides access to the PCIe bus... nothing more... nothing less.
PC users have PCIe slots... so Thunderbolt is much less of a pressing matter.
For any current generation Mac users, Thunderbolt is the only means of accessing the PCIe bus.

Right this moment, you'll go very wrong buying a Focusrite Clarret for use with Thunderbolt under Windows.  
 
With Win10, we finally have "PCIe via Thunderbolt support under Windows".
But... Microsoft chose to support "PCIe via Thunderbolt" (necessary for PCIe level performance) for Thunderbolt-3 controllers that provide Thunderbolt-3 via USB-C port.  
Thunderbolt-2 controllers under Windows do not (and will not) provide PCIe level performance.
 
Quote from the KNOWN ISSUES for the Clarret Windows driver:
"Known issues:- Glitchy audio/enumeration issues when using Thunderbolt 3 connections via an adapter. Please note that Thunderbolt 3 connectivity is NOT yet supported." 
 

To achieve PCIe level performance via Thunderbolt (under Windows), you must meet all the following conditions:

• Running an up-to-date install of Win10
• Running one of the latest Z170, Z270, or X99 motherboards that provides Thunderbolt-3 via USB-C port
• USB-C to Thunderbolt adapter
• Your audio interface driver has to support "PCIe via Thunderbolt" under Windows.

 
 To my knowledge, only MOTU and UA currently have full "PCIe via Thunderbolt" drivers actually available.

Jim Roseberry
Thunderbolt provides access to the PCIe bus... nothing more... nothing less.
PC users have PCIe slots... so Thunderbolt is much less of a pressing matter.
For any current generation Mac users, Thunderbolt is the only means of accessing the PCIe bus.

AAHH.. I see the clouds parting.  Thanks for that..
 

Full bandwidth Thunderbolt support on PC is still very early in development.
If you're going to use a Thunderbolt audio interface (under Windows), know all the details before jumping.