Help with decision for a second army

Most hard hitting: Tau
Most strategic: space marines
Most badass looking: Skitarii

I suggest , if i may, a nice shooty marines list (Iron hands - 1500 pts) allied with a small group (500 pts) of Skitarii. If you feel like a boss, trow in there an Imperial Knight.
This way you have a super fluffy and themed army, good-looking models and not to be underestimated at 1850 - 2000 pts

Slaanesh will punish me for this.

Feel No Pain (6+), or +1 if you already had it. All Vehicles and Characters and vehicles with the Chapter Tactics (Iron Hands) rule get It Will Not Die, and Techmarines get +1 to Blessings of the Omnissiah.
You are looking at regenerating vehicles, extra resilient infantry and buffed techmarines.
They have a strong link with Mars, so fielding with them some rust stalkers or skitarii in general feels good. Also well complemented by an awesome imperial knight.

I'd go for CHapter master on bike with shield eternal, power hammer and artificier armour (2+, T5, 4 wounds, 3++, 6+ Fnp, it will not die....) 5 bikers honour guard with gravguns & narthecium, then some tacmarines with plasmacannons with TL lascannons razorbacks, maybe a unit of three dreads with tl autocannon and the skitarii as you like