ok I'm going to start off with Nvidia graphics cards I would never buy and why: I would never buy a GTX 1070 for these reasons: You can get 2 Rx 480s 4gbs for $300 - $360 any day of the week on Newegg.
a GTX 1060 - You can buy Rx 480s 4gbs for $145 - $180 any day of the week on Newegg.
GTX 1050 ti - you can get RX 480 4gbs for about $150 most of the time or r9 290s for about $120 on eBay or craigslist (I've got an r9 290 for £90 on gumtree before)
GTX 1050s as you can buy gtx 760s or r9 270x's used on eBay or craigslist for like $60 a lot of the time I wouldn't pay more then $70 for one though)
GTX titan xp (just get 2 1080s they'd even look better in a build)
Now for AMD cards: Graphics card buying guide (new and used)
I would never buy a Rx 460 for the unless I can get it for sub $80 new without a rebate for the same reasons as the 1050
I would never buy an RX 470 as for a little bit more you can get an RX 480 or for about the same price a 290
I would never buy anything from amd under the 290 apart from the 270x unless your on a very low budget
Now for new cards, i would buy
I would buy a gtx 1080 as 2 480 4gbs come close you don't have enough VRAM for 4k at ultra on a lot of games and I would just recommend 1 card over 2 unless its amazing deal such as 2 480s 4gb vs 1070
I would buy 1 or 2 RX 480s 4gbs if you can get them for under $180 shipped
I would buy an RX 480 8gb if you do a lot of modded games like Skyrim with a bunch of mods
used cards I would buy: r9 270x or a GTX 760 (if you can get them under $75 shipped r9 290 (if you can get one under $130 shipped gtx 770 4gb (if you can get one under $110) 9800 gt (if you can get one under $20 shipped)
if i missed anything please rip into me and correct anything.