I think I will focus on suggesting pokemon you can catch right now:
If you have enough patience, it is definitely worth it to try and catch a chansey in the safari zone. After evolving that into a blissey (recommend asap since eviolite isn't available until extremely late in the game), basically every special attacker in the game in single battles becomes a non-threat. Unfortunately we don't get toxic TM available, but thunder wave+seismic toss+softboiled is already good enough.
I also would recommend trying to catch a swinub in Evergreen Cave, as it evolves into a mamoswine wh