Solar panel in series or parallel, Which one is better?

It entirely depends on overall system size, solar module output and system optimization between the solar modules and the inverter rating. Typical inverter DC input is limited to 1000V and the solar module serial and parallel connections is determined by this.

Parallel:
If the inverter operates with low input voltage, the modules can be connected in parallel to the inverter, the advantage is that the voltage on the DC side will be lower, safer installation, operation and system maintenance. In parallel the shadow caused which cover the surface affects only one module, this module.

Disadvantage:
low voltages imply large currents and therefore cables of larger diameter, higher cost, or greater electrical losses.

Series:
Transformerless inverters, tends to have higher efficiency, lower cost. With higher input voltages the current generated is low, we can use smaller diameters for spinning, lowest cost.

Disadvantage:
The shading must be considered, since the shadow module that determines the operating current of the entire arrangement in connection series. And have mismatch module losses, due homogeneity power modules resulting in a drop in performance of the inverter.

Generally TL, transformer less inverter we do series x parallel combinations forming strings, > 1KWp

Note: Anyway! Keep all open circuits with no current flow, until all connections are complete to avoid arcing.

If modules are connected in series, then NOMINAL PV voltage increases & current is lower. Thus for same power output, required cable size reduces, resulting in cost saving & improved efficiency due to lower inverter & cable loss. At the same time, inverters & battery ( if used ) will also have to be sized to operate on said voltage. For given storage requirement, It is not advisable to connect batteries in parallel due to circulating current, unless such large capacity battery is unavailable. These inverters operating on upto 760 V or so, are available for on grid installation. For off grid application, inverter operating at relatively lower voltage is used, primarily because very high power requirement does not exist for off grid application.

Thus there is a tradeoff on price, availability, efficiency & safety. All parameters have to be balanced for the right system design.