Do you want prayer or straight atk/str/def levels? That should decide for you. I personally would prefer having more time for prayer so I'd consider hill giants. After the 40s I would consider hill giants as an additional source of prayer and RS Gold flesh crawlers for the reasons previously mentioned. Things won't change much until the mid-fifties, where moss giants may be as an alternate to the hill giants. As you enter the sixties it's time to consider the security spiders' stronghold.
Okay, so I'll need most of the basics, I'll post what gear I'm thinking of using and if someone can edit it for me that would be awesome, and also tell me the method I've got an concept, but having experience will help. My stats are 87 Magic and 80 defense and 83 Hitpoints, if these stats weren't enough, I'd most likely train them. Please let me know.
In addition, I'm aware that soloing the rex can involve killing supreme and prime, so for reference , my level is about 79 (80 easily) and I'm at 82 for Attack and Strength. I'm mostly doing this for cash, so should rex be considered a poor choice for cash any more, it would be nice to know that as well.
If you're a member of the Burstinglobs group chat you'll find almost always someone who is hybridding and is looking for a solo mage companion to kill Rex for them whilst they eliminate Supreme in turn. Prime. This means that you should not have to worry about Supreme or Prime while you're there, except if both of you are terrible at what you're doing.
I'd suggest using Tubby's setup with Mystic Boots and buy runescape levels the Ring of Life until you're confidentenough, and then take a risk and invest in more expensive gear. If there is no one in Burstinglobs looking for a solo mage, just ask if anyone is hybridding/tribridding and can let you hop worlds from theirs, easier than having to kill Supreme/Prime to start your trip.