What's your budget?
Coco de Mer and Je Joue do kits with different sizes and weights to progress to. They're also both made of a beautifully smooth silicone and are a pleasure to use.
The Lovehoney Main Squeeze ones are also good - nice to use and easy to clean, but you have to buy them individually. They also have a lovely silicone covering.
I have the Fifty Shades Beyond Aroused interchangeable ones (£29.99) are good to build up to as you can use them in different weight combos or even without the cradle. For the price, they're excellent for a single one-off purchase. Lelo do a similar system, but they're more expensive.
I have a short vaginal canal so didn't get on with the larger duo balls in the Tracey Cox set. I found it painful. I had the same problem with the Fifty Shades Delicious Pleasure ones (£19.99).
The one that I had the best results with was the Tenscare iTouch. This is amazing and is really actually pleasurable to use - as long as you start slowly and don't zap yourself to high! The problem I had with this was LEADS showing up all the time, though I think they were faulty. When it worked it was brilliant and I definitely had a good result (I have a nerve condition so felt I needed something to keep my nerves stimulated and already use something similar for my leg). If I was buying one again, I'd probably wrap the wires with electrical tape first as they're quite flimsy.
There's also Mystim Truman which has a kegel program and avoids the wires if you want to go the electrical route. He's rather large, though!
New toys on the block are the Impulse kegel rabbit and remote control egg. Again, no wires which is a huge plus.
I'm sorry I haven't posted links but for some reason my computer just doesn't like it. If you search Kegel they'll all come up apart from Truman and the Impulse rabbit.
Good luck and have fun.