In addition to what others have already mentioned for dark elves, high elves and TK, the army with the most effective SC (and SC champions) by far is the daemons of chaos IMHO. Here are a few of the more notable ones:
1) Kairos - very expensive lord level greater daemon with excellent magic (gets to pick up to 8 spells from 8 different lores and knows all the Tzeentch spells to boot IIRC)
2) skulltaker - a popular choice since he can be parked in ANY unit, not just bloodletters, and has heroic KB in a challenge. Better stats & abilities than a khorne herald and cheaper to boot.
3) Changeling - pink horror SC unit champ. Can "swap" S, T, I, & A with any model in base contact. Often used in same unit as skulltaker to nerf anything fighting the unit and then skulltaker crushes the foe who is relegated to T3/S3 with a single attack.
4) Epidemius - usually taken with an all nurgle list as he can boost magic and have poison autowound on 4+ once a few casualties are inflicted.
5) Blue scribes - generates a free random bound spell per turn but most importantly can generate 1 extra power dice for the daemon magic phase for every opponent spell cast the previous turn. All for about the same price as a basic skink priest!
6) The masque - extremely fast hero that can nerf opponent unit's move or LD by D3 each phase. Costs about as much as a naked scar vet and cannot join units but a 3+ ward makes him more survivable. Used in conjunction with the greater icon of despair (lowers enemy LD by 2) this can result in a -5 penalty to LD checks - meaning that even the TG/Slaan bunker is vulnerable!!
Well, that about covers it. Good luck with your tourney.