In terms of speed and reliabiltiy I find EE to be the best. I’m currently on a sim only with them and have EE max (speeds of 90mb/s - often more) 12gb of data with free 4G roaming in Europe and a couple of other countries (like the US). And if you get promotions or your employer has deals with them you can get SIM only contracts for quite cheap. I pay £13 for mine.
Most providers (O2 and 3) only offer 3G roaming so that’s something to watch out for if you travel a lot.
I find that main networks give you better service than MVNOs like giffgaff, but that might just be because I use the services that MVNOs don’t provide like Wifi and 4G Calling.
As an all-round sim only plan I’d probably say 3. Their prices are competitive and customer service is decent.
If you’re looking for data roll on you should check out Smarty. They’re new but they run on 3 and they actually refund you for the data you don’t use. I never used them so can’t talk about experience.
I had a lot of issues with O2 making my experience a hassle and randomly charging me for things and offering poor support when I needed it. Plus I live in Scotland and their coverage ain’t great here. So O2, giffgaff and tesco are all a pass for me.