The consensus seems to be a paladin with massive resists, and big crushing blow through Smite (which ignores defense and always hits). If you poke around here, you'll find more detail - I've been Single Player only since the second season of the 1.10 patch, so can't really help too much. Good luck!
I did uber tristram with passivezon easily. rely on your max valkerie and supporting skills that boost valkerie and for hireling act 3 lightning. You gotta love that chain lightning in uber tristram. Downside your hireling can't kill pit lord but is a great tank. Weapon Tip for hireling is prefreably nothing to do with lightning the sword get other elements. My hireling is dealing chain fire and chain poison on top of his chain lightning.
When interpreting "best" as "most effective", i.e. least investment of resources, time and player skills, then it's a fana-zealot with one point in smie (it's a prereq for holy shield, so he has it anyway). For the Übers, smite is used because you always hit with it and almost all damage versus them is made through crushing blows (from Guillaume's and Gore riders, for exemple), so you don't need its damage bonuses. Versus everything else (except perhaps regular Diablo and Baal), smite sucks and zeal is much better. That's why you don't put more into smite than that one point needed to have holy shield later.
A cheap build has just a Black runeword together with gear that offers crushing blow, open wounds and massive resists. Later on, switching to a Grief phaseblade is excellent because its damage bonus is applied when smiting and this sword is awesome in other melee situations as well. That's mainly useful for life leech (which will be boosted by applying lifetap on the enemies, ionitiated with a charged wand and maintained with Dracul's), but also for the last bit of life versus the Übers.
In my bnet times, my character looked about like this when entering ÜT:
Guillaume's + Um
HoZ + pdiamond
Lifetap wand plus some shield in the weapon/shield switch
Rare dual leech ring, not much mana leech needed
Enough charms for being 125% beyond maximum resists (that's what Meph's aura will deduct). Put away the cube and the tomes and fill up with allres GCs, they have the best performace/price ratio.
Str - enough for gear
Dex - enough for gear and for 75% blocking with holy shield active
Vit - all the rest
Energy - nothing
Apart from prereqs, 20 fana, zeal, sacrifice, holy shield, 1 in redemption and salvation, rest into holy fire (2 points increase max res, without having to activate the aura)
I am quite fond of this write-up, remember basing my tactics on it the first time I went to uber trist...had to retrieve the website from the void.
It shows how uber trist is doable with a VERY budget setup and is both funny and very informative.
I like the idea of killing Übers by whacking them with a bone wand! I would still use a Black runeword instead (and keep a wand with lifetap charges in the back hand), its runes are probably easier to find than Ravenfrost.
The Amazon Basin is still online: however, after a forum merger and upgrades, any link posted before June 2014 is broken (and some of the formatting in the present Soloing Uber Tristram topic has been altered).