eXPlayer
It really depends on what your needs are. I have tried a few video players and all have their pros and cons.
I personally use eXPlayer because:
(1) I can skip forward and backward in a video using the scrubber bar and it always works. For some reason this seems to be a problematic feature in virtually all video players, even the venerable VLC desktop application.
(2) It supports the iPad's hardware video acceleration which provides smooth video and saves a ton of battery use. The vast majority of video apps do not use the iPad's acceleration so they drop frames on HQ video and blow away your battery.
(3) It supports SMB, so simply turn on file sharing for your Mac, PC, or SMB-capable NAS and you can view any video on your networked computer. This was crucial for me. I don't want to have to run separate server/multiplexing/DLNA/uPNP/proprietary apps on the computer; many of these tend to be buggy or have poor performance.
(4) There was a free, fully-featured trial version (limited to 10 mins) so you can test it with your specific files, networks, needs, etc., and know whether or not it will work for you before you pay for the full version. I am very grateful for this because so many video players promise the world but are disappointing.
(5) There is a combo app available so you can buy once for both iPad and iPhone/iPod, or you can save a little money and buy a separate app for just the device size you need. I like options.
The only drawback I can think of is no access away from home. That feature is not important to me, but if it is important to you, you'll want something else.