I see loads of posts about who will be best for modified vehicle insurance.
The truth is that it all depends on you, your location and what you are driving. The best policy for Jimmy on xyz internet forum may not be the best / cheapest for you.
Here's a list of the companies I tried when calling round for cover on my C18XE Corsa.
In no particular order:
Sky Insurance - 0330 333 1250
Greenlight - 01277 376000
Flux - 0800 081 8989
HIC - 0844 888 4888
Keith Michaels - 0800 5422141
Chris Knott - 0844 576 0660 / 0800 917 2274 (Freephone) / 01424 200477 (Geographical - cheaper for mobiles)
Alan + Alan - 0844 5570088
Brentacre - 01792 650 933
Performance Direct - 0844 5733567
Tiger - 0800 803 0451
There are more out there that offer cover for modified vehicle insurance. I'm not google so I'm sure you can search for more yourself.
For a standard car you may find that price comparison sites are your best bet but again, this will depend on you, your car and your location.
Have fun and remember to insure your car correctly. No point telling them it's a 1.0 12v when you have a 300bhp LET under the bonnet.