They'll all very different.
West Hartford is kind of flat (after a couple miles of early climbing), but a good mix of moderate to semi-technical terrain. It's heavily used though, and the trail wear is very obvious. NEMBA does a good job of maintaining it though.
Middlesex is super-technical. Lots of quick ups and downs, lots of slickrock, and very taxing on the legs. Very easy to get hurt at Middlesex if you don't watch yourself.
Cockaponset is great. 30 or so miles of tight singletrack. I stick to a 10 mile loop. Lots of rock gardens, logs, stream crossings, log bridges, and short, steep climbs. It's got some of everything. And it's out in the middle of nowhere, so there's a good chance you'll see no more than a few other riders out. The trails are in great shape.
I'd love to check out Trumbull some day. Like I said, I'm more of an XC guy, so don't expect to see me doing any 30 foot drops.