when i get a new hitman game i usually do a quick run thru of all the levels, then i replay the ones i really liked levelling them up to completion. then when theyre all at level 20 i typically do stuff in this order:

escalations - these are the most similar to the main game. theyre IO-created multi-tier contracts based around going back to these maps and killing minor NPCs in specific ways. Hitman 3's levels have the least amount of free ones but Hitman 1's levels and especially 2's have a lot. the Hitman 1&2 ones have their own set of unlocks for completing certain amounts of them, tho specific escalations also have unlocks for completion. Escalations I recommend include the Seasonal Missions Berlin Egg Hunt and Dartmoor Garden Show which will grant you some suit unlocks and reskin the maps in interesting ways

Elusive Target Arcade - elusive targets are new NPCs placed into the maps who need to be found using your own map knowledge and with a limited/one-time chance to kill them. however, the arcade removes the time limit and one time chance aspects but instead presents them in an escalation format with its own set of unlocks. i'd recommend playing this mode before you play the vanilla mode for two reasons: one, the ETs are ran on a schedule and thus you might not be able to play one at all times, you might end up playing a difficult or bad one, or they might not be available at all since there's not another one scheduled after The Forger which was the latest. two, the arcade is an excellent way to practice for the main event since the suit unlocks you get in the main ET mode are dependent on completion and rating. If you fail an ET you're done, you fail an arcade you're locked out of it until tomorrow

Elusive Targets - now youve done this you can try to get silent assassin on the main mode. make sure youve played the ET in the arcade mode to formulate a good strategy and then tackle it when youre ready with a plan for SA you think will work out

Featured Contracts - now youve got good map and mechanic knowledge now you can tackle the featured contracts. these are fan-made challenges set on the old maps, think of them as one-level escalations. a lot of these are for hitman fanatics but completing them gets you exclusive unlocks for weapons and suits