Do you want prayer or RuneScape gold directly atk/str/def levels? I personally would prefer having more prayer so I would go to mountain giants. Following the 40s I'd say hill giants for additional prayer and flesh crawlers as already mentioned. Things will not change until the fifties when moss giants can be an alternative to mountain giants. Once you reach the sixties you can then also consider stronghold of safety spiders.
Okay, so I want the majority of the fundamentals, I'll post what equipment I believe and in the event that you could edit it it'd be great, and also tell me the approach, I have a vague notion but expertise would help. I've 87 Magic, 80 Defense and 83 Hitpoints, if those stats aren't enough I'd likely instruct them let me know. Also, I am aware soloing rex may involve killing prime and ultimate, so for reference my range level is 79 (80 readily ), and 82 Attack and Strength. I am mostly doing this for money, so if rex is in factn't good cash anymore/ever that would be nice to know as well. Thanks.
Should you apply the Burstinglobs friends chat, there's always someone who is hybridding and needs a solo mage partner to kill Rex about them whilst they kill Supreme and Prime. This means that you don't need to worry about Supreme or Prime whilst you are down there, unless both of you're bad at what you're doing. I'd take Tubby's setup with mystic boots and a Ring of Life until you are confident, then it is possible to begin risking a little more expensive equipment.
When there's absolutely no one in Burstinglobs looking for a solo mage, just ask if anyone is hybridding/tribridding and can allow you to hop worlds out of theirs, simpler than having to kill Supreme/Prime to begin your trip. Bring a tortoise and be certain to have a brew:resurrect ratio of 3:1. If you can, bring 2 fruit bat pouches and scrolls for buy osrs gold safe when your tortoise dies, fruit fall is simply great for soloing Rex, it is not really efficient if you are hybridding/tribridding.