Stock 1.43 O2 UK
Mainly because I'm too much of a wuss to flash a custom ROM.
And O2 UK hasn't released a newer ROM yet.
Although to be honest, theres nothing more than a niggle in the messaging (started after installing a hotfix) that isn't working, and I'm an avid believer of the "If it ain't broke, don't fix it" methods.
Philips Diga -> Philips Savvy -> Nokia 5110 -> Nokia 3310 -> Nokia 3510 -> Nokia 3510i -> Nokia 7610 -> Nokia N80 -> Nokia N80ie -> Nokia E90 -> Sony Ericsson Xperia X1 -> HTC HD2 -> HTC Sensation -> HTC One X (WWE)