My top 5 are:
1. Necron Overlord - Mindshackle scarabs are crazy, no one denies this, the Warscythe does a decent job too. The Necron Overlord has a lot of Wargear buffs, and some nice buffs for Necron units. People dont put their characters / mc's etc over here
2. Ordo Malleus Inquisitor - This guys is a staple of my Grey Knight armies, much like my requirement to have a Dreadknight, without Psychic powers for 80 points he is good in terminator armour with a Psycannon, add in 1 power of Divination and he is very good with Prescience, and even better with Misfortune!
3. Bloodthirster - Only used this guy a few times, but with his flexibility in wargear, the pure terror he causes when he gets into CC, he is one of my out and out top guys. Expensive, but equally when the army decides they have to kill him they are likely to have to dedicate far more than his points to killing him.
4. Vulkan He'stan - Great all round buffer, you like flames and melta? Well this is your man. Hard to deal with in combat with a 2+, 3++ sucks now against 2+ saves, but that balances his amazing buffs.
5. Saint Celestine - All I know about her... annoying. Slay the Warlord can become impossible, and she is more than capable of wiping out entire units herself in combat!