While any class can use them the two I hear most about are the Wereform druid and the Sorceress. I think there's a guide floating around in the Sorceress forum as there's a few ways to play her out. Oh, and I just remembered about a third, the melleemancer. I suppose a charger could also use one.
An asn can use a cruel war pike (spear) or an elite polearms class wpn (unique or with a godly runeword in it) without really actually using the wpn itself: my trapper has such a wpn on switch when she needs to use BF for helping with her merc/shadow killing LI monsters in Hell. The only problem is how many stat points you want to invest in her str to pick up a highest possible dmg stick!
I'm not sure if there are any chars that consider polearms as their BEST weapons, but i can tell you for sure they are the most used merc weapons. also, my LLd pally used a polearm+charge to finish of any runners or wussy duellers, even though he is a zealot