I don't own an iOS device personally, but I know of a few games which are on both Android and iOS which I have enjoyed.
All of my views on these are based on the android versions and they are not listed in any particular order.
1.
Magicka: Wizards of the Square Tablet ($2.11):
This is quite a fun, cute, colourful game where you can just mess around and if you're mean, you can kill your friends
It's an RPG-brawler with spell casting and you can make your own spells and up to 4 player co-op. It has got cross-platform co-op but it's not perfect. The humor and gameplay kinda makes up for the short-ish playtime imo, but it's great for the price tag.
2.
Worms 2: Armageddon ($4.99):
If you don't know what the Worms franchise is, then I highly suggest checking it out. The mobile version is very similar to the original PC version, with some more features added ontop. There's multiple different modes too, online (I cannot remember up to how many players it is), local hot-seat (2-4 players) and single-player. For the price tag, you get hours of gameplay but it does require a constant internet connection.
3.
Asphalt 8: Airborne (Free):
This is one of my favourite racing games on mobile. I know mobile driving games isn't for everyone, but if you have any interest in racing games at all, then I suggest checking this out. The driving controls are stable and feel good to use, and the steering and breaking feels like it has some weight to it, so it feels satisfying when you win. The learning curve is perfect, and the difficulty goes up at a decent speed. Multiplayer works well too, I don't know how racing with friends works though, but I believe it's possible. For a free game, of course there is micro-transactions, and they are kinda shoved into your face a little bit, but it's not a P2W game since most of it you can get from earning currency and such. I've not brought anything in the game at all, yet I still have a lot of fun playing it. I will note that, on Android at least, the game is BIG. Like, over 1GB total.
I'm just going to add these two since I cannot make this post without adding them
(For these, I do not know the price in USD unfortunatly)
4.
Minecraft: Pocket Edition (£6.99):
You would have to be living under a rock these days to have not heard of Minecraft. The mobile port of this game is now pretty stable, even though it's a bit behind the PC version. There's many mods, maps and skins you can put into the game, and if you're a coder but want something easy, then the language that MC: PE is coded in is a lot simpler then PC Minecraft! With both multiplayer and single-player, you can explore both alone and with friends, or join a big server and play some minigames. Imo, making your own server on MC: PE is a lot easier then doing it on PC, but you still might need to google for a tutorial on how to do it.
5.
Terraria (£4.99):
Terraria is often compared to minecraft as it being "Minecraft's 2D sister". I much prefer Terraria over Minecraft since it has so much content, and the mobile version has unique content that PC/Console don't even have! As for the multiplayer side, I'm not exactly sure how it works for iOS as I believe it is (Or at least, used to be) different to how it works on Android. But apparently there are mods which allow you to do online play either way. Of course, there's some content on PC/Console that isn't on mobile, but with the stuff that mobile adds, I believe it to be a fair trade. Of course, community mods and maps exsist for the mobile version too!
Sorry for the long read but I hope these reviews helped you. I don't know very many good, free multiplayer mobile games since most of them in my experience are MMOs and that kinda thing, or P2W (Both of which, I really don't like personally).
