Warpriest is among the best armors in F2p but it's only available to members... except if you were a participant in the event in the past, that RS Gold I don't think. It maxes out at tier 2 in F2P (was modified by Jagex) and is not the greatest armor in most conditions.

It's possible to still find decent hybrid armor. the ghost hunter armor (w/e it's known as) is as excellent and you can obtain it by fighting ghosts. There's also dragonstone armor however, it's expensive. I believe it was maxed out at level 50. Best hybrid armour f2p based on what it was described by him as dragonstone which is tier 60.

It is also worth noting that a World Event is coming this summer (May or June I believe) therefore you should be able grab some warpriest then. In the past, when you were able to receive warpriest was around 2014. I'm talking about tier 2 or 3 tiers , or whatever. I'm not familiar with the terminology it was just a 07 player dropping down. Warpriest Armour is a shambles F2P. It's tier 50 however its stats are lower than similar tier 50 "tank' armours due to being hybrid armour.

Warpriest is only useful only if you are doing a lot of PvP multi-teams with clans or teams it's not worth the hassle for those who don't (except the cape). Regarding the topic's question like others have already answered Batwing is the most effective Magic armor for F2Pers. But you can upgrade your weapon(s) If you're currently using Batwing with a book or shield.

Gravite staff is significantly better and OSRS Fire Cape is definitely worth a look if you don't plan to join in the near future. It's a Dungeoneering bonus worth 40K tokens. To get it, you'll have to get 400K exp through regular training in the Dungeoneering skill.