I second the recommendation of air video. If I'm looking to strem something off of my MacBook or other NAS device directly to my iPad it works like a charm.
I would also second the recommendation for A/V player. It's always works quite well for me streaming everything extremely well and even high definition MKV files fairly well most the time. Every once in a while I'll get a hold of an MKV file that is a little cruddy – but like someone else mentioned earlier I think it's generally the fault of a poorly compressed file rather than the AV player program. I do think it's kind of silly to want to spend time doing anything to a file I download off the Internet so my advice is from the perspective of somebody that has no patience for "fixing" a video file on my PC I find off the Internet. My expectation is that the video player should work for a wide variety of files - and AVPlayer fits the bill.