By - tongole4
Though I'm not really sure what faction to recommend, I would recommend the 'Battlescribe' app. It allows you to build an army list using any of the factions. Great for theory crafting and experimenting.
The interface takes a bit of getting used to but I feel it's simple to use.
Its free although a subscription is the price of a coffee for a whole year.
I am still new to the game so take what I say with a grain of salt but guard seems to have lots of customization both in the selection of units they have and the amount of units they use on the table can lead to some interesting combos I think. Also space marines have a huge amount of options and can do anything and everything.
Tau has many weapons and systems that you can customize based on your need.... flamer, burst cannon, plasma rifle, cylic ion blaster, airburst, etc. Not to mention the half a dozen systems that you can attach. Crisis suits have 3 hard points and Commanders have 4, so you can imagine the possibilities when you have a squad of 3+ Crisis Suits and one or two Commanders.
Just make sure you magnetize!
Space Marines are probably the deepest and most flexible by some margin.
I'd say dark angels, you can play them as battleline marines only, give them support from ravenwing (custom bikes) or deathwing(extra beefy terminator) and with our 9th codex we now can get CP back for ravenwing and desthwing detachments you could even do all terminators or bikes, or mix the two! All terminators isn't the best but nothings more fun than going onto turn three only having lost like 5 models in the first two turns.