Dromons, Riders, Musketeers, Enkidus, Swiss Merc. Out of all units in the poll I only like Immortals (saves 10 sp), and Legionaries (a Pike with +2 attack).
I hate Ancient UUs. The Enkidu is the exception, since it mainly helps against Barbs, and prevents the AI from attacking. Immortals and Legionaries (you can build them until you have a GA) are units I use in early medieval, with Trebs as back-up. Dromons are simply great for about 2 eras.
Since I rarely fight backwards CIvs with Cavs, the Cossack isn't very useful for me, and I don't like the GA timing (late MA is fine, but to use Cossacks/Sipahi most efficiently, you need to beeline to MilTrad).
Sipahi is too expensive - at that time, I usually lack raw numbers of units, not quality (hardly ever attack something not red-lined).