I would choose Nibros. The tactic helps to effectively reach ranged units with dogs and jumping demons. In logistics, you only need to upgrade the fast portal opening skill. It's harder to "get" that ability early on with Grog. Plus, +1 to luck is also good.
Nibros is good if you're not planning to give him the "Call of Urgash" ability.
Delab is good, of course, with his ballista; it's unlikely anyone will beat Delab with a fully upgraded ballista.:smile38:
Marbas' "Spellbreaker" is especially good against Mages and Dark Elves.
On particularly long maps, it's better to play as Grog "Swiftfoot," and, as the name suggests, at levels 25-30, he can move for a very long time.:smile13: