It really depends on your style. I have several C&C games, and played them extensively, years ago, but even with the release of C&C RA3 3: Uprising from the bundle, I've only played it once. Instead, I am finding myself wanting to play Starcraft 1 OR 2, Warcraft 1 OR 2 or Age of Empires 2 OR 3. Warcraft is my overall favorite, but sadly I don't own them anymore, so when I got AOE2HD, which is fantastic, I was thrilled.
Those three series are probably the easiest of all the RTS ones, simply because of the interfaces and the huge community that you simply won't find anywhere else.
That being said, here are some other recommendations.
Knights & Merchants - Kind of an AOE/Settlers Mix
Settlers - Just look for it here and buy it (You won't be disappointed)
Warlords Batlecry 3 - Many people recommend the 2nd, but I love the 3rd, it is all in all a great game, with more focus on Combat than Building.
EDIT: OH and find Dune 2000, there is (Or was) a website that lets you play it online. It is very similar to C&C Red Alert, but just pure awesome.
EDIT 2:
chezybezy: i second Age of Empires too, still not sure if its worth getting the HD version on steam, ill prob end up picking it up when its on offer knowing me.
It totally IS worth it. It's the AOE 2 that we know and love, in HD. Though the term HD really means more zoomed out, but it does have full widescreen/eyefinity support. And it has full Steam Workshop integration, which makes it INCREDIBLY easy to try out mods and texture packs and more. Plus it is getting a new expansion, I believe.