57 Votes in Poll
Easiest to paint I'd say Necrons... If you're not looking at putting in too much work you can just spray them silver, put on a shading wash of Nuln Oil and use a bright green like Warpstone Glow to pick out the eyes and any other details... you're done
For guard on the tabletop know that HQ and commisars are a must, and to always remember you have orders. You have the best selection of armor in the game try to use it. Don't hold your ground, your guys are ass in melee unless you build them with that in mind. I might be a bit of an extreme example but anything with "does not require line of sight" is a good thing to bring, going from mortars to basilisk's. For the Emperor!
I play both Space Marines and Imperial guard, imperial guard is very expensive because of how many models you have to field in order to play decent sized games but are easy to paint and their tactics are easy to learn. Space marines are cheaper cause of less models are flexible you can mess up with them in a battle and still win but are debating harder to paint
The guard needs a lot of important units on the board to be efficient.