Controllers depends on personal preference really. If you want to use it like a proper gaming tab then the Ipega 9023 is your only real option, not got one myself but I have the 9021 and apart from the dpad it's very good. If you're using it primarily for emulation and have the tablet on a stand then I think the 8bitdo controllers are great. I have the FC30 Pro and it has a great dpad (seriously, it's fantastic!) and nice mini analog sticks as well as two pairs of shoulder buttons. It can link via bluetooth as a generic pad but also via USB as an XBox360 pad which is great for Windows games, so it's pretty much the pad to use if you need something compact to play your Steam/GoG library on the go.
One drawback with the 8bitdo is the lack of analog triggers, so for racing games it's not ideal. In fact due to the small size it's not great if you need to use both sets of shoulder buttons in a game, I have big hands but can usually get used to small controllers yet I find it a struggle sometimes. I would say if you want to play mainly PC games then get a wired or wireless XBox360 pad. Drawbacks here are the dpad, overall size and stupidly long cables - however you can shorten the cables if you can solder, I did that on the USB dongle for my wireless pad but I guess it's the same procedure for the wired version. I wish they would integrate the dongle into the tablet, would make it a much better experience.