I'm at a toss up between RTS and 4X TBS.
One the one hand, RTS has a pacing issue against you. Everything is on a time, and you need to move quickly or else you'll fail. TBS doesn't deal with this, and you have as much time as you need usually.
That said, the level of depth is hugely different.
RTSs play quite differently to each other, but one constant is the same as RPGs: Builds.
In general, you pick a build at the start of the match, begin that build whilst quickly scouting your enemy, and then judge which builds your enemy is not suited to, and make sure you're suited to countering all other builds.
Early game in RTS, after a little bit of practice, is fairly easy.
Mid game is largely micromanagement and needing to multitask. Build and expand, scout the enemy, and ensure your troops fight well by microing them. This becomes reasonably easy once you get the hang of hotkeys and number groups, but is probably the hardest part of it overall.
Late game, IMO, only really appears when two good players [Or equally matched, I guess, sometimes] face off against each other. Otherwise the game is usually decided in the mid game, and its just a cleanup afterwards. Late game is probably the most tactical part of the game. You've done your set build. You've survived and expanded, and now your economy is on the decline as resources run out, whilst rapid skirmishes deplete your military faster than you can rebuild. It becomes a game of bluffs in some ways, as well as fast reaction and good judgement. Its a juggle of balancing your expansion so that you can get enough resources, with your military so that you can defend them, and you can't just look up a build order as how you do that changes depending on the outcome of the midgame.
So, RTS games do have tactical skill and depth, though early on that's just abstracted behind simple builds and such. They also have a time scale.
With 4X games, you often can't have it go past in real time, as you need all that extra time to manage everything else in your empire. The number of interplaying systems, the match size, types of resources, technology tree - a whole different level to an RTS. Whereas RTS games have a lot of simply designed builds, and those builds are easy to find due to the low number of interplaying systems, 4X games somewhat transcend that. Not to say they don't have builds, as that'd be a false, however its a lot harder to come up with those builds. Especially where maps are randomised rather than standard symmetric arena maps with fixed starting positions, certain builds can become entirely useless based off the terrain around you. Some competitive games would have arena styled matches, others realise it takes a lot of the strategy out of the match and instead use a game's often inbuilt 'balanced start' function, where it will try to balance resource allocation around each player, but still provide a random terrain such that neither player can simply rely on a pre-determined build. Because of this, a lot of builds will often have very vague guidelines rather than "Turn 10, build this" like you get in RTSs. It'll be "Build this building sometime when you can, because its the best building for helping you get to your victory style", or "This is the optimal research tree to get this technology first", however stipulations have to be given for if you have a town that has very low production, or food, and needs those facilities built more urgently than your victory facilities, or if you need a certain happiness technology more at this point in order to keep half your cities from revolting.
Hell, a big debate was had about whether to move your starting unit first turn, or settle down ASAP for Civ. People can't even agree on what to do in the first turn, let alone the rest of the game.
The number of routes to victory also complicates things. Whereas an RTS is victory only through destroying the other player's buildings entirely, RTS have conquest, diplomatic [Usually discounted as it doesn't work competitively], technology, cultural, domination...
And it can be a lot harder to tell what build your opponent is going for. Far from just sending a worker over to their base and microing it around so its not instantly killed, or sending a cloaked/fast flying scout, RTS games often have dedicated mechanics for finding out information on the other player, involving spies and another form of resource related to spy tasks. Simply seeing an enemy's base isn't enough, as all that you know from that is that they have a base.
All of this complicates matters such that it is very difficult to know what your opponent is doing, and needing to plan ahead can lead to greater difficulty in reacting to the opponent's plans.
So, I'm kind of torn between the two. Personally I find greater difficulty overall in 4X games, but both are worthy contenders.
