The optimal meta list can change at anytime. Repointing every 6months, FAQ, new battletomes that shape the meta, etc.
2 general rules I find helpful are:
1) Buy models you like.
2) Have the goal of collecting 1 of each kit in the range and put it on your shelf painted.
But as others said... start by buying 2 start collecting boxes for IJ. 6 pigs with the warchanter buff and some ardboyz to fill out an ironfist will be solid stuff for a long time.
Pick up a Megaboss on foot next probably (or Mawcrusher... but understand he might die in a lot of games and if that makes you mad/sad since he costs $$$/points maybe wait a bit or leave out completely).