Best SM tank?

Khan on Bike
Bike Command Squad (all with Meltaguns, Storm Shields, and mixed LClaws and THams)

3x5 Bikes with Double Melta+PFist
2x10 Tactical marines with RBack, Missiles, Melta's, Combi Melta + PFist
2 LS with MM/HF
Trip Vindicators (nude)

Counters for pretty much everything, and that command squad is hard as nails.

The Tactical Squads are meant to secure your home objective, and the two Razorbacks make up for the Heavy Weapons you're losing out on by not taking a further two tactical squads. If you take a Rhino, you can't afford a further two Heavy Weapons, so you have a choice - Combat Squad both, and run and the Melta+Sergeant in the RBack to take on enemy walkers or annoying units (I use Lascannons or TL HFlamers), or just sit both squads on an objective. Most people know that 20 T4 3+ Wounds are hard to take out, especially in cover, when there is a killer melee squad, multiple melta's with an effective 18" melta effect (12" move + 6" melta + 6" standard pen), and triple vindicators trundling around.

With Rhino's, sure, you can transport the squad, but that's not what they're there for. If you need the transport (say against Eldar), then CSquadding Meltas and putting them in a Las/LasPlas Razorback gives you reasonable maneuverability, and is an effective counter to Grav Tank Armies.

3 Vindicators, Dreadnoughts and Mech marines not competitive?

Can I please come to live under your rock?