Are you fed up with Orange/EE because of coverage issues? Customer service? Slow internet speeds? Knowing a bit more on this would provide more targeted answers here.
Best thing to do is to get a free Sim card from other networks that'll allow you to check what coverage is like. I won't include anything for Orange/EE as I'd assume you're already familiar with it...
Vodafone -
O2 -
Three
Dolphin PAYG on Orange, as I'm sure you know, gives you 400 texts and 1GB of mobile data for topping up £10 in the previous month and you keep that £10 for credit. Bear in mind that many suggestions like those given here for giffgaff and Three mean offering a bundle of calls, texts & data that is deducted from your balance either when you activate it, if you have it "queued up" to start when the current bundle finishes, or when the next renewal of the bundle starts, not as an additional extra for topping up, so make sure that you're aware of this and note the difference.
You're not limited to just those networks - there are numerous "Virtual Operators" that act as their own network but use the network infrastructure of a "real" network. For example, Virgin Mobile uses EE, Mobile by Sainsburys uses Vodafone and Tesco Mobile uses O2. So for example Tesco Mobile will work anywhere where O2 has a signal for you. The majority of virtual networks are on EE's network, the most well known among them being Asda Mobile, Vectone, & Virgin, while Lebara and Talk Mobile use Vodafone's network with Lycamobile and giffgaff using O2's network. There are a couple of virtual networks on 3, but these are aimed mainly at mobile data users.
I'll leave it to others to add what they think is "best" - quite often personal circumstances will come into play where some deals look good but might not suit yourself, and vice versa.