Our  preference historically on visits to Smogland is to catch the train to Darlo, have a few pints there (as often as not in the legendary Hogans), then get the last possible train over to Boro. This costs a bit more than if you go direct all the way through. And if you do opt for this then if you're not careful you'll end up getting caught up in a very slow escort from the station to the ground and may miss the kick-off. If on the other hand you decide to go straight through to Boro then your choice of pubs is very limited. I once wrote that Watford is a desert when it comes to decent pubs. While this is true, Watford is a veritable oasis compared with Middlesbrough. Just about the only place that gets half decent reviews is Doctor Browns. As it happens this is the second closest pub to the ground, and it's normal for away fans to get in there. On the down side, being handy for the ground means it gets very busy, particularly towards kick-off. In terms of alternatives, you can forget all those around the station as they're not safe. A bit further afield in the Isaac Wilson (map), which is a Wetherspoons. You should be ok in there with no colours on, and they serve a good pint. A bit more out the way and therefore probably safer again is The Resolution (Lloyd's No 1).  Last time we played Boro the Bridge Inn was designated an away fans pub. This obviously meant it was safe, but the place is a dump and absolutely heaving.
Pub map

The Football and Real Ale Guide
Internet Ground Guide
CAMRA pub guide for Cleveland (not many Middlesbrough pubs though!)