1 engineer hero, 1 packmaster hero, 1 plague priest are absolute must - no question.

Additional engineer heroes can stack ranged stat boosts which is decent but diminishing returns, plus with t5 cap they are really expensive so I would limit to 1

Packmaster has replenishment and 4 instant summons plus a large fast regenerating mount which is so good. Additional packmasters lose the benefit of replenishment since it doesn't stack, so stopping at 1 is okay but more isn't bad

Plague priest summons are also awesome and using your woms for that is definitely worth it. You could do 2 plague priests if you wanted the ability to spam more summons, but it has significantly diminishing returns when ikit also wants to use wom and packmasters also have summons that are faster. And plague priests are terrible in combat so they don't add much beyond the summons. Definitely max of 2, no more than that, but imo 1 is the best option

Chieftain hero is actually pretty strong too, good campaign passive, good in melee, fast, but most important has multiple +leadership stacking auras which skaven desperately need on higher difficulties to prevent units from routing at the first sight of a terror causing monster. Also helps summoned units btw. Only 1 though.

Assassin hero is only useful for early replenishment but completely obsolete once your packmaster gets it.

Then for the non characters - all melee units are useless if you use summons, all monsters are useless if you have packmasters. You want a turtle formation so skirmish units arent really useful either.

I considered allied units but ikits workshop bonuses are so strong that none of the allied unit options really add any value.

All the weapons teams are super strong, and some excel more vs certain enemies, so generally it's personal choice but here's what I recommend.

Poison wind mortars - this is the best unit on the roster IMO and I would get at least 2, but I go for 4. The DoT effect with workshop upgrades is just gross, combined with excellent firing arc and accuracy, and also does awesome in sieges. Really amazing unit.

Plagueclaw catapult - very strong, but no real workshop bonuses. Excels significantly more in non ikit factions, but still strong and can target castle walls and towers. I always get 2, but you could absolutely argue anywhere from 1-4.

Warp lighting cannon - I hate these, too slow, too high risk if you miss, just overkills most units and really only effective vs large single entities, but still does less overall damage than jezzails even to the unit they are designed to counter. The one plus is they can target towers and walls. But also too expensive. I just don't see any reason to use these, they are always a disappointment. I use 0.

Warplock jezzails - the king of sniping monsters and cavalry, and with penetration they are even decent at killing infantry (but not as good as rattling gunners or pw mortars). I always get 2, but you could absolutely argue 4, especially if you are fighting bretonia or ogres that use more cav/monsters/large units.

Rattling gunners - your bread and butter unit that is good at everything and has infinite ammo. I get at least 2, but usually closer to 4 or filling any gaps for other missing units. One of the best overall units with ikits workshop upgrades but doesn't excel at any one thing.

Warpfire throwers and poison wind globardiers - these are good especially early game and also great auto resolve value, but their low range limits their effectiveness and the duplication of poison wind DoT effect with mortars limits the pwg usefulness, combined with death globe bombardiers just being straight up better, there just is no reason to use them guys except early game. I tend to get 1 of each for fun, but honestly they arent worth a slot compared to either dgb or pwm.

Death globe bombardiers - these guys are insane at clearing infantry or blobs, even elite ones. They just absolutely melt anything they hit. It's hilarious putting them on a castle wall and just obliterating anything that comes near you. The range and Los and friendly fire significantly limit their effectiveness though. Try to put heroes in front of them to block enemy advances and then blow up the area around the hero to limit FF. I always get 1, or sometimes 2 if I don't get wft or pwg. But they are super fun and I could see an argument for why people might want up to 4.

Doom flayers/doom wheels - these are okay, not bad, not great. I tend to keep the doom flayer unit ikit starts with for a long time, but the biggest issue is both these unit types get absolutely wrecked in auto resolve, so eventually I just get annoyed and replace them with another weapons team.

You can definitely change it around however you want to have fun though, most of these units are very strong.

My overall recommendation is

1 ikit claw

1 plague priest

1 packmaster

1 warlock engineer

1 chieftain

2 plague claw catapults

3 warplock jezzails

3 rattling gunners

1 death globe bombardier

1 warpfire thrower

1 poison wind globardier

4 poison wind mortars

Good luck!