That's a tough call... I don't have any really nice, high-end mech mods or anything. I usually use a few different mods at the time, with different flavors in the tanks of each one, and switch a couple of times throughout the day (I like most flavors, at least to a degree, but there aren't but a couple of "ADV" flavors I've found so far, and I still get tired of them after a day or so, if I don't mix it up some, so I typically have a few setups going at once, with a different juice in each one). I'd say that my favorite mods that I've used are either the Wismec Predator, Wismec Reuleaux RX 2/3, or Smok Alien-- but there are several others I like too. Those are probably the ones that get used the most though-- and if I need something more compact I'll usually grab the Smok Stick V8-- and actually I just got a Smok QBOX that's pretty awesome. It's the smallest mod I've ever seen haha. It's about the size of one of those little miniature candy bars.
I've been looking at some rdas and rtas lately, trying to decide which way to go-- I'm still just using the pre-made, factory sub-ohm coils exclusively. Are you happy with the Goon? And I hadn't seen the AE Duke before just now, but it and that Tesla nano are both looking pretty sweet.