It would be best to go to IKEA and test the chairs yourself, because everyone's body shape/build is different.

Many people recommend the Markus (599, 10yr guarantee) but when I tried it, was a little uncomfortable for me.

The Matchspel (699, 3yr guarantee) felt pretty comfortable and the dimensions were just right. I'm still considering getting this, only thing I am wary of is the PU seat. IKEA also knows that PU leather can only last about 3 years. Mesh back, so rather cooling.