Which water heater types do you recommend?
When most people think of a water heating system, they think of a tank (40-75 gal. on average) with a heating source.  However, a robust water heating system probably shouldn't be so simple.  How you want to configure things depends on what your exact situation is, and what features you want maximized.  Some examples:

2 tank, parallel
This kind of setup gives you the storage capacity of both tanks (say, 2 x 40gal = 80gal).  But it also costs twice as much to keep running.  I can't imagine why you would do this over the next idea, but I'm sure someone has a reason.

2 tank, serial
This gives you a lot of versatility in water availability and cost.  In low demand situations, turn the first tank down or off.  Storing the water will let it come up to room temperature, usually reducing the heating requirements in the second tank.  Set tank 2 for your desired temperature.  When demand increases (say, relatives visit for a week) turn the first tank up to boost the inflow temperature.  Ultimately, both tanks at the desired output temperature will give you slightly more hot water than the capacity of both tanks combined; due to heating as water flows.

Tankless
As mentioned by others, a simple tankless heater has hidden drawbacks in demand-based activation and flow restrictions.  However, the do not heat any water unless on, so their standby energy cost is nil.  Installation can be tricky: they require costly ventilation and exhaust systems which can make them not cost-effective over their projected lifespan.  And they are difficult to tie into hot water recycling systems, if you're looking for instant-hot water at the tap.

Tank + Tankless
You're getting into the area of boilers, and options for combining things do very different things.  A pre-warm tank can improve the flow rate of a tankless heater, by reducing the required temperature rise.  A post-heat tank can alleviate the demand-based problems, but adds the cost of continual heating and complexity in cycling cooling water from the tank.

Other
As I mentioned there are thousands of potential ways to combine systems.  Gas is cheaper than electric.  Tankless doesn't have standby costs.  Tanks smooth out warm water flow.  You have to determine your situation, and what works best.

Suggestion
It may be against the TOS to talk brands, if so someone remove this or let me know.  Since we're building a house this summer I've been looking into this problem, and our contractor pointed out the Eternal brand of tankless water heaters.  They have a 2 gallon internal tank that they keep warm, which supposedly fixes the problems of demand-based activation and still costs very little for standby heat.  They integrate well with recirculation systems, and can even be combined in multi-unit setups if demand exceeds one unit.  I've not yet played with it (still building) but the concept looks great on paper.  Oh, and they have less expensive exhaust requirements as well.