You get the same dictionaries on the basic Kindle (Kindle 4) than you do on the Kindle Touch or the newly released Kindle Paperwhite.
None of the e-readers meet all your requirements; none will easily read all formats, have good dictionaries and have page turn buttons.
However, if you remove the read all formats requirement (as HarryT says you can convert your books and instead choose the e-reader that most suits you rather than your ebook library), you do have better options.
HarryT has already covered the fact that if good dictionaries are important to you, then Kindle & Sony are your best bet, so with page turn buttons that leaves you with the Sony PRS-T2 or the basic Kindle.
The Sony PRS-T2 is a touch screen device, which makes it probably best for looking up the dictionary, it's very quick & has an excellent screen, the page turn buttons at the bottom of the device aren't best placed though (IMHO).
The Kindle isn't a touch screen (so you have to use the five-way controller for things like dictionary look-up), not quite as quick as the Sony for dictionary usage but page turns are speedy, and again it has a very good screen (buy the black version and not the silver to guarantee this, it seems far better). It has the page turn buttons handily situated on the side, which is most comfortable for one handed reading.
The Sony PRS-T2 is the better of the two, but it costs USD $130 compared to the cheapest Kindle option of $70.