Normally I'd agree with the choice of Excadrill, Ferrothorn or Krookodile since they're overall really powerful Pokémon, but in this particular case maybe a Flying- or Psychic-type Pokémon could be better, considering you don't have anything for Elite Four Marshal (the strongest E4 member, in my opinion).
On the upside, Flying-type Pokémon give you the option to use Fly, which is a very good utility move in the overworld. The less good thing is that Flying-types are much more scarce in Unova than in most other regions, and the regional bird is absolutely terrible. The only ones that are really good are Braviary (which you can only catch right before the eighth gym and evolves WAY too late to be useful) and Archeops. Archeops can be obtained by backtracking to the Relic Castle, talking to a Backpacker there and receiving the Plume Fossil. After that, bring it to the Nacrene Museum and talk to one of the receptionists. Exit and enter again with an empty slot in your party and you got Archen. Archeops has really high stats and optimal coverage. Alternatively, you could also obtain the 8th badge first. After that, head through the gate that leads to Route 10. The receptionist will warn you about a storm on Route 7. Go to Route 7, enter one of the houses in the middle of it and talk to all the people inside. After exiting, you will see Thundurus outside briefly. From that moment onwards, Thundurus will start roaming Unova, and while it takes a decently long time to hunt it down and capture it, Thundurus is a pretty powerful Pokémon to have with you.
As for Psychic-type Pokémon, there are three rather noteworthy ones- Beheeyem, Musharna and especially Reuniclus. All three suffer of acute 'I-have-no-speed'-itis, but there are some differences. Musharna focuses mostly on bulking attacks, having higher HP, Defense and Special Defense stats than the two others. On the flip side, Musharna also has the lowest Special Attack. Beheeyem has high Special Attack, rather good Defense and Special Defense stats, but suffers from a low HP stat for a bulky Pokémon. Last but not least, there's Reuniclus, who is in the middle- it has the same Special Attack as Beheeyem, a little lower HP than Musharna, the same Defense as Beheeyem and slightly lower Special Defense as Beheeyem. Reuniclus is arguably overall the best of the trio, since it's the best at combined bulk with power. However, the other two should work just fine as well.
Another possibility I nearly forgot about is Sigilyph, a Psychic/Flying-type. Sigilyph has decent Speed, Special Attack, Defense and Special Defense, and is overall a well-rounded Pokémon. It could work as well. If necessary, there's also Swoobat, which is also Psychic/Flying, but this one suffers from having high Speed but mediocre everything else. There are also Unfezant and Swanna, but please don't use these, they're terrible compared to all the abovementioned Pokémon.
Should you want something that isn't a Psychic- or Flying-type, however, abovementioned Excadrill, Ferrothorn or Krookodile work like charms. Other possibilities that have worked really well for me in the past are Stoutland, Gigalith and Conkeldurr, although the latter two require a trade to evolve.
/rant