That's a good question. My favourites change all the time. As at right now, my favourites are:

Favourite 'bot modes:

PotP Optimus Prime - It looks like MP Optimus Prime without the $200+ price tag, and being able to look like a well-proportioned and heroic-looking OP while accommodating the Evolution gimmick is quite amazing.

Generations Thundercracker/Dirge/Thrust - The best G1 Seeker mould ever! Robot modes are well-proportioned and accurate to the animation models. Best part, there's no cheating in the transformation and the cockpit kibble on the chest really turns into the jet cockpit.

Favourite Alt-modes:

TR Skullsmasher/Krok - I'm a big fan of beast modes, especially those released in the Prime Wars Trilogy. The neck and jaw articulation are simple features, but they add so much character to the figure. You can do some really dramatic poses and expressions that you can't do with a vehicle mode. With Skullsmasher/Krok; you can have a relaxed looking-crocodile when you close the mouth, you can have an angry/growling crocodile when you open the mouth wide and tilt the head looking up, or you could have a confused crocodile by rotating the head sideways and leaving the mouth slightly ajar. Some of these expressions can be quite funny.

PotP Sinnertwin - Similar reasons as Skullsmasher/Krok above, but I also like alt-modes that are interesting and unusual. There's nothing quite as unique and weird as a Transformer that turns into a two-headed dog.

Generations/TR Cosmos - It's a very simple-looking alt-mode, but I find it amazing that a well-proportioned robot mode could transform (and almost shrink) into a small, flat, predominantly disk-shaped object. There's almost no kibble in either mode (unless you count the thrusters on the robot's back). I guess I'm really more gushing over the transformation than the actual alt-mode.

Those are my favourites, I'm sure it'll change tomorrow