iTeleport (Jaadu VNC) is the best in my honest opinion, after trying several others over the years.
The current version from Jaadu is for the iPhone and they are soon to release the iPad version for free to those who already owned the program prior to April 3rd. You can purchase the iPad version at any time though and be charged $25 app price for it, even if you already bought the previous version on iPhone, or wait for the free upgrade path in app store for existing iPhone version owners.
They also have a Windows RDP version called Jaadu RDP, its also $25 but between these two apps, it's the best $50 I spent on my phones. The Jaadu RDP is still being updated as well but wont release until after they get the VNC version up. This is knowledge after just speaking to them about the updates via e-mail.
Both programs are very robust and still work on the iPad as the iPhone versions. I can't wait to install the iPad version, so tempted to pay the app price again, but will probably wait for the free upgrade which is very soon.
You can't ask for better support either, they are only two Devs on these apps, but are tentative on their forums as well as e-mail.
These two programs are my bread and butter on certain days. $25 a piece is a small price to pay for such good polished apps and I applaud them for their continuous updates as well.