There isn't really a comparable series to Dark Tower that I know of - it's pretty unique. The first time through the first four books I loved it - but then re-reading them, and the final three that he wrote years later, I couldn't really stand it. But I know what it is about and what you are looking for, and it's really hard to think of something comparable to the way he brought in Western elements, the Wizard of Oz, etc. The idea itself is a very cool mash-up, and I don't really know of anything too similar.
There are plenty of epic fantasy series, though. I'll list some off the top of my head.
Tad Williams - this one is quite similar to Tolkien
- Dragonbone Chair
- Stone of Farewell
- To Green Angel Tower
(There is a follow-up series that isn't finished yet, but you can wait on that)
Piers Anthony - this one is kind of more similar to mythology (its own mythology) and is more cheeky/humorous, but it's great
- A Spell for Chameleon
- The Source of Magic
- Castle Roogna
- Centaur Aisle
(There are tons more in this series)
Robert Jordan - this one is so frustrating, because he lost control of the plot about seven books in and died without finishing, like we assume George RR will, but a lot of people like this one. The female characters are a total unmitigated disaster. I'm only mentioning it because my quibbles might not matter to you, and it is epic.
- The Eye of the World
- The Great Hunt
- The Dragon Reborn
- The Shadow Rising
- The Fires of Heaven
- Lord of Chaos
- A Crown of Swords
(Plus seven more)
Brandon Sanderson - he is the guy who had to come in and finish the above series. He is great and has a bunch of different series. You could start with Mistborn.
- The Final Empire
- The Well of Ascension
- The Hero of Ages
(Those are the ones in the series I have read, but there are more. I have liked everything I have read by him.)
The Song of Ice and Fire series that Game of Thrones was based on is awesome, obviously, but the stupid bastard is never going to finish them, and the TV series ruined them in many ways.
There are tons more, but these are some ideas of things you could try.