Easily RotF Leader Starscream. Accuracy, interesting transformation, fun gimmicks (love the flip out weapons!). That retailed at the time for the average leader class price.

I have the MPM-01 redeco MIB. I bought it at the time it released. I can't remember what the RRP was at the time, though I know on the secondary market now it does fetch quite a good price. I also have the GDO/Generations (can't remember which subline) G1 repaint, though again can't remember the RRP or how much I paid for it.

A shout here for ROTF Leader Optimus Prime (and the myriad repaints thereof), but Starscream pips him to the post for favourite figure ever.

As for super duper expensive stuff, I'm a big fan of Fansproject's take on Menasor. That whole set ran close to $500 USD at the time, and while it doesn't fit in aesthetically or size wise with other combiners (3P or otherwise) it's still my favourite.

While we're talking 3P I love Planet-X Vulcun (FOC Grimlock) to bits. I think that ran me about $120 USD at release. Worth it though.

As for average priced stuff, here are a smattering of my favourites from voyager and below, in no order of preference:

Deluxe
Titans Return Triggerhappy
Titans Return Wolfwire
Prime First Edition Starscream
Prime First Edition/Generations GDO Cliffjumper
Generations Drift
Generations WFC Optimus Prime
Transformers Animated Rodimus Minor
Universe 2.0 Cyclonus
Hunt for the Decepticons Tomahawk
Classics 1.0 Mirage
Transmetals Tarantulas
RotF Sidearm Sideswipe
Movie 2007 Dropkick
Movie 2007 Longarm

Voyager (or equivalent)
Generations Springer
Revenge of the Fallen Mindwipe
Dark of the Moon Human Alliance Roadbuster
Revenge of the Fallen Human Alliance Jazz

These are the figures that more or less constantly reside in the toybox in my living room for idle fidgeting while watching movies.