Internet wisdom seems to be Tzeentch for the Lvl4s and Nurgle for lower level (1-2).
Slaanesh is a pretty lacklustre lore all round. And you really want the benefits of the MoT so there’s not THAT much going for the unmarked sorcerors. A lvl4 DP with Tendrils is a “reliable” caster of Gateway.
snowblizz, that’s a fairly good build for it, however, it needs to get Gateway and it costs 510points minimum. Davespaceman has only 1.5k left and wants to squeeze in 4 sorcerers. With more points, it sounds good, but with the wishes, I think something a little cheaper wouldn’t go amiss.
Try a level 4, with mark of Tzeentch, conjoined homonculus, skinhidden plate and blood of Tzeentch, costs 400 points (it will almost always cast and has the toughness boost to help it stay alive.)
Just realised how expencive he is, but he’ll get the job done. Keep him protected as well.
Second, take another level 4 with mark of Tzeentch. Give him the skull of katam and keep sorcerers near to him (preferably the one above). Add the Rod of Torment for an extra spell each turn and surprise your opponent. Totals 385 points. Again, expencive, but h will boost your other sorcerers and gets a sneaky spell. Keep him protected as well.
Take a Level 2 Chaos Sorcerer, mark of Nurgle, book of secrets and power familiar, plus bloodcurdling roar. 210 points. He will cast a lot of spells, (thinking about it, keep him near the above one as he’ll benefit more than the first one) has a sneaky fire/shadow/death spell (I prefer fire) and gets a bonus power dice to use. He also has the usefulness of a shooting attack. It’s strength one, but 2D6 no armour saves, it’s basically 6s to wound up to T4 (goodbye Chaos Knights).
Another level 2, mark of Tzeentch or use fire lore, infernal puppet and, if you wish the bronze armour of zhrakk (keeps him alove a little longer). Third Eye of Tzeentch can be good hear, to use some unexpected spells against your opponent. He costs (assuming all upgrades suggested) 215pts and has a few tricks to play.
That’s 1,220 points so far. With the rest take some warriors and give them the Banner of Wrath: extra magic missile each turn is always good and it can be a parent unit to one or two of these sorcerers.
Hope the input helps or inspires.
Also, since I said it was too expencive and then totalled almost as much, the DP is still a very good idea: it has better stats and would have a 4+ ward, helping it to stay alive.
The one problem with DPs is that they are vulnerable to shooting as they can’t hid in units. One build to conside ris sticking your sorcerer on a chaos steed and giving him ablative hound armour (I.E. put him in a disposable warhound unit so he can’t be picked off) any shot that do hit him will have to get past a 2+ save and with the chaos steed’s attacks as well he won’t be a pushover for normal troops in combat. You could even stick him in a unit of horsemen or knights.
As far as lores go IMO fire and shadow are the best depending on whether you want a purely destructive or more complimentary style of magic user. The two movement spells in lore of shadows are both very useful and underestimated. Steed of shadows is great for eliminating warmachine crews or plucking a character out of danger. In the good old days of sixth edition you used to be able to cast steed on daemon princes and exalted daemons - 40" of flying move in a turn on a 4+ cast! I used to upgrade my killy DP to a level one sorcerer purely to get this spell - there was nothing like it for pouncing on enemies’ flanks in turn one.