As stated, the Sipahi (a very dominant unit because of the ease of obtaining its required tech early in an age) does not get the absolute best combat bonuses. For instance, the Iroquois get a jump from 2/1/2 to 3/1/2 (and in an age when the best non-UU defender is a 2). The Greeks start off with their UU and (in C3C) don't have to worry about barbarians causing an early GA. In addition the Greek UU is the best defender until musketmen come in to play. Additionally, the hoplite costs the same as its counterpart.
The Roman Legionairy also has a 3 defense, although, since it costs 50% more than the hoplite and lacks a good upgrade until Industrial Age, there are some weaknesses. Another weakness of the Roman unit is its dependance on iron.
The Immortal used to be, IMHO, the best UU since it dominated two ages (almost), but since the creation of Berzerkers and Medieval Infantry, the Immortals are Immortal only in one's memory.
One of the previously worst UU's that is now fairly nice is the musketeer. In the past, it went from 2/4/1 to 3/4/1 (which really sucked!). Now, it goes to 2/5/1 with a bombard! Stacks of musketeers can really do a number on many units of their age. In addition, musketeers can be kept around in the early IA to do the exact same thing (i.e. have, albeit slightly less powerful, defending artillery-type units before the advent of guerillas and , later, TOW Infantry).
