The low cost lightweight superzooms - The two Tamrons, the two Sigmas, the Nikkor 200-500. When I was looking at getting one, I looked at all except for the Tamron G2 since it was not available at the time (summer 2016). I have a 600mm F4 to compare with, and was looking for something that came close. The Sigma Contemporary and the original Tamron weren't even close. The Nikkor was pretty good, but the build quality was not what I was looking for - I needed to get a lens that I could use in snow, mist, drizzle and light rain. The Nikkor didn't offer that, and it was less sharp at 500 than at around 450mm, and it was much sharper stopped down to F8. When I attached my camera to the Sigma Sport I was very surprised at how good it was. Wide open at 600mm it was very good, and by F7.1 it was excellent. So I bought one. I've since shot with a borrowed Tamron G2, and had it been available in 2016, I would have gotten it if only because it was 2 lbs lighter. Similar image quality as the Sport, good build quality, very manageable. Sorry to say, the two Nikkors I had tried just didn't cut it on image quality or build - and at the end of the day it was still only a 500mm lens. The G2 and Sport were for all intents and purposes as good as my 600mm F4, with the exception that I can use the 600mmF4 in really crappy light.

I take it you are using a crop sensor camera. No, the 18-300 will disappoint. Either the G2 or the Sport will not. The 200-500 will have you wondering what you are missing.

I would avoid anything grey market - it's nothing but trouble when it breaks or fails.