Personally, my favorite chips of the premium brands are Xecutor 3, followed by SmartXX, followed by Xenium. Unless you're going with a solderless install, in which case I'd go with Xenium, follwed by Xecutor 3, followed by SmartXX.
The thing that sets Xecutor 3 apart in my mind is all the optional products that they've released that work seamlessly with their chip, like the X3IR which allows you to turn your Xbox on and off with the controller or remote, which is really fantastic if it's going to be a media center in addition to a game console. They also have the X3 LCD which is a nice plug & play LCD that you can cut a hole in your case and install, or you can just use their replacement front-panel with a premade hole that the LCD fits right into. The SmartXX chip is just incredibly solid and uses some encrypted chip or something that has made it impossible or impractical to crack. The upside is that to this day there has never been a SmartXX clone, where with Xeniums, if you buy from the wrong dealer, you could end up with a cheap ripoff that WILL get fried and stop working.
I've had all three chips and they're all solid, but my favorites have been the SmartXX and Xecutor, even though I've installed a lot more Xeniums, cos when I install mod chips for other people I'm uncomfortable soldering them because the extent of my soldering skills comes exclusively from the two Xbox's I've done a solder install on. So, since Xenium has the best solderless solution that's what I've used most. It's a fine chip . . . just not my favorite. And the guy who runs the team Xodus, who make Xenium, is really really obnoxioius.