Ooh there's so many I could recommend! It really depends what you want. If you want something cheap and not very valuable to mess around with, go for a Lumia 640 or 640 XL, you can run WP8.1 and W10M and it'll hurt at lot less to brick it than bricking a 1020 or a 950. The Lumia 950 and HP Elite x3 are the ultimate Windows 10 Mobile devices, W10M has the best app support but it does feel a bit less cool imo than WP8.1, and even the final release is buggy. If you want something a bit nicer with the classic Lumia pillow shape and an OLED screen, get a 735 (or a 730 which is the 3G version), WP8.1 looks GOURGEOUS on this phone and they go for pretty cheap on ebay just like the 640. If you want a giant Windows phone for old WP games and watching movies and stuff, the 1520 and 1320 are the 2 giant Lumias, they're a bit more expensive but still super cool (no OLED sadly). My personal favourite is the Lumia 1020, with the insane 41MP camera, and I LOVE the yellow, but these are very rare and very expensive, not really one you want to mess around with. It also didn't officially get W10M, and runs it pretty badly unofficially. The Lumia 830 is also pretty cool, it's mainly just the design, but just like the 930, the 830 has the same issue with the buttons that fall out very easily. If you can find one, the Lumia Icon (also known as Lumia 928) is absolutely beautiful, but very rare just like the 1020. The HTC Windows Phone 8X comes in some pretty neat colours too!