If you are looking for a Space Marine army that can rapidly and reliably put close combat units in the enemy’s face, then a solid option is to take a White Scars “Scarblade Strike Force” detachment that includes the following:
-Battle Demi-Company (TDA Chaplain, 3 x Tacs with Grav Cannon, ASM with double Flamers, Devastators with quad Grav Cannons, all in Drop Pods)
-1st Company Task Force (3 x TH/SS Assault Terminator squads)
-Land Raider Spearhead (3 x LR Crusaders with MMs)
Three of the Tacs/ASM/Devs arrive by Drop Pod Assault, filling the enemy DZ with targets and firepower Turn 1. Simultaneously, TDA Chaplain and Assault Terminators ride in the LR Crusaders and move up as fast as possible… that translates into a 12″ move, 6″ Flat Out, and another D6″ Flat Out because of the Scarblade Strike Force “Ride the Wind” special rule (so a total of 19-24″ move across the board toward the enemy DZ).
Additionally, as long as the LR Crusaders stay within 6″ of each other, they ignore all Shaken, Stunned, Weapons Destroyed, and Immobilized results due to their “Armoured Behemoth” special rule. This, combined with their extra movement, all but ensures that they will get across “No Man’s Land” safely and in good shape to deliver their Assault Terminators into combat Turn 2. Once the Terminators do get into assault, they benefit from Fear, Fearless, -2Ld to enemy units within 12″ of all three squads, and Preferred Enemy against a selected enemy unit (chosen before the game begins). They also have Hit and Run at I4 (re-rollable, due to Scarblade Strike Force rules), which makes them even more potent and flexible in combat.
Even the mightiest Tau gun-line and the most elusive Eldar Jetbike Host will have trouble stopping a force like this, due to its enhanced mobility and survivability, not to mention the raw power it packs once the Assault Terminators actually get into combat.