Starbases, like anything, really depend on your situation and strategy, and whether you're fighting the AI or human. Building a starbase is ordinarily a huge decision, but they're easily the most powerful battlestations in the game. They're effectively impregnable in the early game. With a small fleet to support them and 3 or 4 upgrades, it can be nearly impossible to take a starbase down. On the other hand, starbases can't phase jump, so if you put it in a bad location the enemy can still ravage the rest of your empire.
This is especially true in multiplayer where human players will simply look at a powerful upgraded starbase and keep going. They'll attack softer targets, and if your fleet is much smaller there's not a lot you can do about that. As a result, starbases need to be placed very well to have any overall benefit, and won't save you at all if your fleet is substantially smaller. However, the AI is prone to just charge them head on (a death sentence) so building one or two on the front lines is a great idea in those situations.
Well, there's the Envoy Cruiser, the Construction Cruisers (build starbases), the Assault Cruisers (anti-structure), and the minelayer cruiser. Envoys are purely diplomatic, and construction cruisers are only for building starbases, so I'll skip over them. The assault cruisers are for TEC and Advent. The TEC Ogrov is a great anti-structure weapon that can rip through starbases quite nicely due to its long range and high damage. The Advent Adjudicator is less useful, since it's more expensive and packs less of a punch. It's generally regarded as a useless unit since it's too fragile for its high cost.
Now every faction has minelaying capacity. The TEC build them like tactical structures, and so they don't have a dedicated minelayer unit. In general, this means TEC has to set up their mines long in advance since their construction frigates have to meander about and build each mine individually. The Advent carrier doubles as its minelayer, but this really consumes lots of antimatter so it's not very effective. The Vasari have a dedicated minelayer, and it's simple to use with no strings attached, easily the best minelayer and the only type of mines that I ever bother to use. However, its only purpose is to lay mines.
Entrenchment has brought a new world of strategy to multiplayer. For singleplayer, it has its upsides and downsides. On the one hand, it's got lots of new awesome features. On the other hand, the AI hasn't taken well to starbases and I've found the difficulty has never been the same as a result. The AI is particularly bad about spending too much money on starbases and gimping its fleet (thus screwing over every planet not protected by a starbase, leading to defeat) or charging headfirst into my own starbase and getting its fleet ravaged.
The Kol is still underwhelming. Definitely in the bottom 5 capital ships, arguably the bottom 3. The Sova is pretty awesome now, and the Akkan has been much improved, so TEC hasn't been completely ignored, just the Kol.
